goodwe-addon/goodwe_controller/app/arbiter.py

"""Who gets to command the inverter, and why.

Every strategy - the grid-following loop, the maintenance cycle, peak shaving,
the safety stops - stops writing the setpoint directly and instead returns a
CLAIM each cycle. This module resolves the claims into one number.

⚠️ THE REASON THIS EXISTS. Two controllers writing one actuator is the failure
this whole system is built to avoid: it is what the vendor controller did to the
reference site, and §4.4 of the project notes calls it out by name. Before this
module, "exactly one writer" was a convention held up by an if/else ladder and
careful reading. With an EV charger and a heat pump eventually wanting the same
battery, a convention is not good enough - so precedence is now a first-class
object with one rule and a printable explanation.

THE RULE, in full:

  1. The highest-priority `set` claim wins. If there is no set claim at all, the
     target is 0 W - fail toward inaction, never "hold the last value".
  2. Every `limit` claim whose priority is >= the winning set's priority is then
     applied. Most restrictive wins.
  3. Contradictory limits (lo > hi) are a BUG, not a tie to break: command 0 W
     and say so loudly.

Clause 2 is the interesting one. It is what makes "money outranks maintenance"
a consequence of the priorities instead of a special case somebody can forget:
the maintenance charge-only limit binds the loop, but a higher-priority peak
shaving claim simply is not bound by it.

⚠️ AND WHICH ACTUATOR IS A CLAIM ABOUT? THAT QUESTION HAS AN ANSWER AGAIN.
specs.md §6.1 declared `actuator: str` as the first field of `Claim` and the
implementation shipped without it, so every claim was implicitly about "the one
and only actuator" and `resolve()` returned one scalar. ARB-03 restores the
field, because §6.2 decided the site grid target is ALLOCATED across several
inverters and never broadcast to each of them - and an allocator needs a seam to
occupy that a single scalar does not have.

So resolution is now THREE STAGES, and they are separate functions so that a
test can construct a case at any one of them:

  1. `pre_resolve()`  - each inverter's OWN claims become an interval for that
                        unit. One unit's claims, one unit's bounds.
  2. `site_resolve()` - the SITE claims become one grid target S. This is the
                        rule above, unchanged, and it is deliberately the same
                        code it was: at N=1 there are no unit claims, so this
                        stage sees the whole list and answers exactly what the
                        shipped arbiter answered.
  3. `allocate()`     - THE SEAM. It turns S into per-inverter shares subject to
                        stage 1's intervals. ALLOC-01 filled it: the allocator
                        itself is `app/allocator.py` and the reasoning lives
                        there, because it consumes intervals and a number and
                        must never learn about claims, priorities or P1.

⚠️ THE STRUCTURAL PROPERTY, AND IT IS THE REASON FOR THE SPLIT. A per-inverter
constraint is consumed by the allocator as an INTERVAL; it is never promoted
into the site claim list. So a unit `hi` and a site `lo` are never terms of the
same `lo > hi` test, and the contradiction clause below - which resolves to 0 W,
a battery parked inside a billed quarter - cannot be reached across two
inverters. That is DECISIONS/SAFETY-25-loop-soc-floor.md §0c, made mechanical:
not "guarded against", NOT EXPRESSIBLE. Filing a unit constraint as a site claim
to "keep it simple" is the whole bug, and `actuator_for()` is what stops a
strategy doing it by hand.
"""

from dataclasses import dataclass, field
from math import inf

# ⚠️ ONE WAY ONLY. `allocator` knows nothing about claims, priorities or
# this module - it consumes intervals and a number - so the dependency
# cannot become a cycle and the allocator stays testable on its own, which
# is what lets test_alloc.py sweep it hundreds of thousands of times
# without building a claim list for each one.
from . import allocator

# Priorities live here and nowhere else. The moment these become magic numbers
# scattered through the strategies, the whole point of this module is lost.
P_SAFETY = 100        # stopped, inputs missing, shutting down, device limits
P_MAINTENANCE = 60    # maintenance owns the actuator outright (drain, hold)
P_PEAK = 50           # capacity-tariff peak shaving - costs real money
P_MAINT_SHAPE = 40    # maintenance shaping the loop (charge-only)
# OPT-04. §10.1 priority 3 - price arbitrage. ⚠️ ITS POSITION IS THE SPEC, NOT A
# preference: P_PEAK (50) is ABOVE it so clause 2 lets the capacity-tariff floor
# clamp an arbitrage charge, which is §9.1's "give peak shaving priority over
# arbitrage whenever the two conflict" expressed as arithmetic rather than as a
# rule somebody has to remember. P_MAINT_SHAPE (40) is above it so a speculative
# spread cannot override the pack's balancing cycle. P_LOOP (10) is below it
# because trading against the meter is the entire point - the grid-following
# loop would otherwise cancel every charge the moment it created import.
P_ARBITRAGE = 30      # price arbitrage - trades against the meter on purpose
P_LOOP = 10           # ordinary grid-following

SET = "set"
LIMIT = "limit"

# --- actuators, and the two scopes a claim can have -------------------------
# ⚠️ §6.2: THE GRID CONNECTION IS AN ACTUATOR TOO. It is the one every inverter
# at the site shares, and two inverters regulating against one meter are two
# writers on it - which is the failure this module exists to prevent, one level
# up from the one it was built for. So it gets a name, and claims about it are
# filed on that name rather than on nothing in particular.
ACTUATOR_SITE = "site"

SCOPE_SITE = "site"     # about the grid connection: one of these per site
SCOPE_UNIT = "unit"     # about one pack, one inverter, one device

# ⚠️ THE CLASSIFICATION, AND IT IS THE REAL OUTPUT OF ARB-03. Each entry is read
# off what the claim is ABOUT at its construction site in main.py, not off which
# module builds it. Getting an entry wrong is not cosmetic: a per-inverter
# reserve filed as a site `hi` against the peak guard's site `lo` is `lo > hi`,
# clause 3, 0 W - the battery parked in the billed quarter that SAFETY-25's memo
# calls fatal. The reasoning is recorded here, at the line, because this table is
# where a ninth source will be classified and the reasoning is what a future
# reader needs in order to classify it correctly.
CLAIM_SCOPE = {
    # --- SITE: about the grid connection ---------------------------------
    # main.py: `Claim.set("loop", P_LOOP, decision.target_w, ...)`. compute()
    # regulates `grid_w` - the P1 measurement - to zero. Its subject is the
    # meter, and §6.2's whole architecture is ONE integrator on P1 whose output
    # is then split. A site claim, and the one the split exists for.
    "loop": SCOPE_SITE,
    # main.py: `peakguard.claim(...)`. The quantity is the 15-minute average
    # OFFTAKE AT THE GRID CONNECTION, which is what the Belgian capacity tariff
    # bills. There is one quarter-hour peak per site, not one per pack.
    "peak": SCOPE_SITE,
    # main.py: `arbitrage.claim(...)`. It buys and sells at the grid against a
    # day-ahead price. The price is a property of the connection, and the energy
    # it trades crosses the meter.
    "arbitrage": SCOPE_SITE,
    # main.py: `Claim.set("safety", P_SAFETY, 0.0, "stopped" | "inputs missing")`.
    # ⚠️ READ BOTH CONSTRUCTION SITES BEFORE MOVING THIS ONE. Neither is about a
    # pack. "stopped" is the operator switching control off for the SITE, and
    # "inputs missing" is the SITE's own telemetry gone - the meter this whole
    # controller regulates against. Both must stop every inverter, and a site
    # SET at P_SAFETY is how that is said once rather than N times.
    "safety": SCOPE_SITE,

    # --- UNIT: about one pack, one inverter, one device -------------------
    # main.py: `Claim.limit("device", P_SAFETY, self.dev_min, self.dev_max,
    # "rating")`. The numbers are read off ONE inverter's own reported min/max.
    # Another inverter has another rating; there is no site rating.
    "device": SCOPE_UNIT,
    # main.py: `Claim.limit("supervised", P_SAFETY, -max_w, max_w, "max_w")`.
    # `max_w` is the supervised cap on what this controller may WRITE to a
    # setpoint entity - §11.3's "the actuator is capped below the inverter
    # rating". It bounds a write to one device, like `device` does and for the
    # same reason, so it is scoped like `device`.
    "supervised": SCOPE_UNIT,
    # main.py: `Claim.set("failsafe", P_SAFETY, 0.0, "sign disagreement")`.
    # ⚠️ THE EVIDENCE IS SITE-WIDE AND THE REMEDY IS NOT, and that is why this
    # entry needs its reasoning written down. SAFETY-02 compares P1 - a site
    # measurement - against `standing`, THE COMMAND THIS CONTROLLER PUT ON THIS
    # ACTUATOR. What it concludes is "the unit I am driving is pushing power the
    # wrong way", and what it does is park that unit. At N>=2 the same evidence
    # cannot be attributed to a sibling nobody here commands, so a site-wide
    # remedy would be this watchdog stopping an inverter it never graded.
    "failsafe": SCOPE_UNIT,
    # main.py: `Claim.set("maintenance", P_MAINTENANCE, ...)` when the cycle owns
    # the setpoint, and `Claim.limit("maintenance", P_MAINT_SHAPE, hi=...)` when
    # it only shapes. §12's monthly recalibration is a PACK procedure - drain
    # this pack to its floor, charge this pack to full, recalibrate this pack's
    # coulomb counter. Two packs recalibrate on their own schedules.
    "maintenance": SCOPE_UNIT,
}


def actuator_for(source: str, unit: str, units=()) -> str:
    """Which actuator a claim from `source` belongs on, given the site's roster.

    ⚠️ ROUTE THROUGH THIS, DO NOT PASS `actuator=` BY HAND. An unclassified
    source raises `KeyError` here rather than defaulting to the site, which is
    the one direction that is not safe to default in: a per-inverter constraint
    that lands in the site claim list is exactly the `lo > hi` this card exists
    to make unreachable. A ninth claim source has to be classified in
    CLAIM_SCOPE before it can be filed, and test_peakguard.py part 10f is the
    other half of that - it goes red on the line the ninth append is written.

    ⚠️ AND AT N=1 EVERY SOURCE ROUTES TO THE SITE. That is not the classification
    being ignored, it is §6.2's identity: with one inverter the fabricated grid
    reading and the battery setpoint are the same number, so the grid connection
    and the only unit ARE the same actuator and there is nothing to allocate.
    Splitting the list at N=1 would be a behaviour change on a live safety path -
    the `failsafe` and `maintenance` SET claims would stop competing with the
    site's for the winner, `explain()` would name a different source, and
    `loop_is_following()` reads that name. The identity holds only at N=1, so
    this function is the ONE place that stops relying on it, the day the roster
    grows.
    """
    if CLAIM_SCOPE[source] == SCOPE_SITE:
        return ACTUATOR_SITE
    return ACTUATOR_SITE if len(units) <= 1 else unit


def scope_violations(claims) -> list:
    """Claims filed on an actuator their source's scope does not permit.

    A list of one-line complaints, empty when the routing is right. This is the
    check `actuator_for()` cannot make on its own, because a caller can always
    write `actuator=` by hand: it grades the claim list that actually reached the
    arbiter, whatever built it.
    """
    out = []
    # ⚠️ THE SECOND TEST IS CONDITIONAL ON THE SITE HAVING NAMED UNITS AT ALL,
    # and that is not laxity. At N=1 §6.2's identity holds and a per-inverter
    # source filed on the site is CORRECT - it is what `actuator_for()` returns.
    # The direction that produces `lo > hi` only exists once there is a second
    # actuator for the constraint to have belonged to.
    split = any(c.actuator != ACTUATOR_SITE for c in claims)
    for c in claims:
        scope = CLAIM_SCOPE.get(c.source)
        if scope is None:
            out.append(f"{c.source}: not classified in CLAIM_SCOPE")
        elif scope == SCOPE_SITE and c.actuator != ACTUATOR_SITE:
            out.append(f"{c.source}: a SITE claim filed on unit {c.actuator!r} - "
                       f"it would bound one inverter's share instead of the grid")
        elif scope == SCOPE_UNIT and c.actuator == ACTUATOR_SITE and split:
            out.append(f"{c.source}: a UNIT claim filed on the site while the "
                       f"site has named units - it can cross a site limit")
    return out


@dataclass(frozen=True)
class Claim:
    source: str
    priority: int
    kind: str = SET
    value: float = 0.0          # for SET
    lo: float = -inf            # for LIMIT
    hi: float = inf             # for LIMIT
    reason: str = ""
    # ⚠️ specs.md §6.1 DECLARES THIS FIELD FIRST AND IT IS LAST HERE, ON PURPOSE.
    # The spec's point is that a claim NAMES its actuator; the tuple position is
    # not the point, and it is load-bearing for two things that are. Every
    # construction in this repository passes `source` first, and
    # test_peakguard.py part 10f reads `Claim.set(...)`/`Claim.limit(...)`'s
    # FIRST POSITIONAL ARGUMENT off main.py's AST to pin the claim-source set -
    # the tripwire that guards SAFETY-25's deferred option. Moving `source` off
    # position 0 would silently blind that scan, which is this repository's named
    # failure mode. So the field is restored where it costs nothing.
    #
    # ⚠️ AND ITS DEFAULT IS THE SITE, EXPLICITLY. Not None, not "". A claim that
    # does not name an actuator is a claim about the grid connection and says so;
    # "unspecified" is what the shipped code already had, and it is what made
    # every claim implicitly about the one and only actuator.
    actuator: str = ACTUATOR_SITE

    @staticmethod
    def set(source: str, priority: int, value: float, reason: str = "", *,
            actuator: str = ACTUATOR_SITE) -> "Claim":
        return Claim(source, priority, SET, value=value, reason=reason,
                     actuator=actuator)

    @staticmethod
    def limit(source: str, priority: int, lo: float = -inf, hi: float = inf,
              reason: str = "", *, actuator: str = ACTUATOR_SITE) -> "Claim":
        return Claim(source, priority, LIMIT, lo=lo, hi=hi, reason=reason,
                     actuator=actuator)


@dataclass(frozen=True)
class UnitBounds:
    """What one inverter's OWN claims say about that inverter's share.

    STAGE 1's output, one per unit, and the thing ALLOC-01 consumes. It is an
    INTERVAL plus the two facts an allocator cannot reconstruct from an interval:

    * `pinned_w` - the value that unit's own winning SET asked for. A SET is not
      a bound: `resolve()` CLAMPS a set value into the limits rather than
      intersecting with it, and folding a 2500 W drain into `[2500, 2500]` would
      turn "clamped to the rating" into "contradicts the rating". So it travels
      beside the interval, not inside it.
    * `gate` - the priority of that winning SET, or None. Clause 2 applies a
      limit only at or above the winner's priority, and at N>=2 the winner may be
      a SITE set chosen at stage 2, i.e. AFTER this. Stage 1 states the
      constraint and records what it is gated on; deciding whether it applies is
      stage 3's business. Guessing here is how a maintenance charge-only `hi`
      would come to bind a peak discharge it does not outrank.

    `contradiction` is a unit contradicting ITSELF - its own `lo` above its own
    `hi`. It parks that unit and says so; it does not park the site, which is the
    entire difference this card is about.
    """
    lo: float = -inf
    hi: float = inf
    lo_src: str | None = None
    hi_src: str | None = None
    gate: int | None = None
    pinned_w: float | None = None
    pinned_by: str | None = None
    contradiction: bool = False

    def explain(self) -> str:
        if self.contradiction:
            return (f"CONTRADICTION within the unit ({self.lo_src} lo="
                    f"{self.lo:.0f} > {self.hi_src} hi={self.hi:.0f})")
        span = (f"[{'-inf' if self.lo == -inf else format(self.lo, '.0f')}, "
                f"{'+inf' if self.hi == inf else format(self.hi, '.0f')}]")
        if self.pinned_by is not None:
            return f"{span} around {self.pinned_by} -> {self.pinned_w:.0f} W"
        return span


@dataclass
class Resolution:
    target_w: float
    winner: str                     # which source set the value
    bound_by: str | None = None     # which limit actually changed it, if any
    contradiction: bool = False
    considered: list = field(default_factory=list)
    # --- the three-stage fields. ⚠️ APPENDED, AND DEFAULTED, ON PURPOSE: every
    # existing construction of this type is positional in its first five fields
    # and `target_w` still means exactly what it meant - the SITE grid target.
    # main.py assigns it to `self.target` unchanged.
    unit_bounds: dict = field(default_factory=dict)   # stage 1, per actuator
    shares: dict = field(default_factory=dict)        # stage 3, per actuator
    allocated: bool = True          # False when stage 3 declined to split
    scope_errors: list = field(default_factory=list)  # mis-routed claims
    # ALLOC-01, appended for the reason the four above were: `target_w` still
    # means the SITE grid target and every existing construction is untouched.
    #
    # `allocation` is stage 3's full record - what each unit was told and why,
    # what could not be placed, what was dropped. `shares` and `allocated` are
    # two fields OF that record, kept at the top level because main.py and four
    # test files already read them there. ⚠️ IT IS THE RECORD, NOT THE PAIR,
    # THAT SAFETY-28 GATES THE WRITE PATH ON: `allocated` says the allocator
    # stands behind the split, and `unmet_w` says how much of the site target it
    # could not place, and fusing those two would make one saturated inverter
    # park the whole site.
    allocation: object | None = None
    # The priority of the SITE claim that chose `target_w`. Stage 3 weighs a
    # unit's OWN winning SET against it - see `allocator.plan()`'s PINS note -
    # and getting it from here rather than recomputing it is what stops the two
    # rules drifting apart: this is set by the same line that picks the winner
    # `explain()` names.
    winner_priority: int | None = None

    def explain(self) -> str:
        """One line an operator can act on - this is the point of the module."""
        if self.contradiction:
            return f"CONTRADICTION between limits - commanding 0 W ({self.winner})"
        if self.bound_by:
            return f"{self.winner} -> {self.target_w:.0f} W, limited by {self.bound_by}"
        return f"{self.winner} -> {self.target_w:.0f} W"


def pre_resolve(unit_claims: list) -> UnitBounds:
    """STAGE 1. One inverter's own claims -> that inverter's interval.

    Clauses 1 and 2 of the rule, transposed: the unit's highest-priority SET
    names what the unit wants and gates which of the unit's limits apply, and the
    most restrictive of those limits gives the interval. Read UnitBounds on why
    the SET value is not folded into the interval.

    ⚠️ THE INPUT IS ONE UNIT'S CLAIMS AND NOTHING ELSE. A site claim reaching
    this function would become a bound on one inverter's share - the mirror image
    of the fault this card removes, and the reason `resolve()` partitions rather
    than filters twice.
    """
    sets = sorted((c for c in unit_claims if c.kind == SET),
                  key=lambda c: c.priority, reverse=True)
    gate = pinned_w = pinned_by = None
    if sets:
        gate, pinned_w, pinned_by = sets[0].priority, sets[0].value, sets[0].source

    lo, hi = -inf, inf
    lo_src = hi_src = None
    for c in unit_claims:
        if c.kind != LIMIT or (gate is not None and c.priority < gate):
            continue
        if c.lo > lo:
            lo, lo_src = c.lo, c.source
        if c.hi < hi:
            hi, hi_src = c.hi, c.source
    return UnitBounds(lo, hi, lo_src, hi_src, gate, pinned_w, pinned_by,
                      contradiction=lo > hi)


def allocate(target_w: float, bounds: dict, *, weights=None, live=None,
             scope_errors=(), site_priority=None) -> tuple:
    """STAGE 3. THE SEAM, AND SINCE ALLOC-01 IT IS OCCUPIED. `(shares, allocated)`.

    ⚠️ THE ALLOCATOR ITSELF IS `app/allocator.py` AND THE REASONING LIVES THERE.
    This is the adapter: it keeps the two-value shape every caller in this
    repository already reads, and it exists as a separate name so that the
    arbiter's three stages are still three functions a test can enter at any
    one of them. `allocator.plan()` returns the full record - use that when you
    need `unmet_w`, `dropped` or the per-unit reasons; `resolve()` stashes it on
    `Resolution.allocation` for exactly that.

    What §6.2 fixed and what this delivers:

    * SUM-INVARIANT. `sum(shares) + unmet_w == round(target_w)`, in integer
      arithmetic, so it cannot drift. Rounding is largest-remainder because a
      per-unit `round()` biases the sum by up to N/2 W every tick and at 1-2 Hz
      that is a PERMANENT standing error the site integrator chases forever.
    * EACH UNIT IS TOLD A FUNCTION OF ITS OWN SHARE, never of the shared
      measurement. That makes the gain at the meter a convex combination of the
      per-unit gains for ANY split, which is why the weights are a comfort
      choice and a bad split cannot destabilise the site.
    * WITH AT MOST ONE UNIT IT IS STILL A PASS-THROUGH, byte-identical to what
      ARB-03 shipped: the share IS the site target, as a float, unrounded and
      unclamped. §6.2's N=1 identity, and the compatibility promise every
      existing replay in this repository depends on.

    ⚠️ AND IT STILL DECLINES, WHICH IS NOT THE SAME LIST OF CASES AS BEFORE.
    ARB-03 declined at N>=2 because it had no allocator; that trigger is gone
    and the branch is NOT. It now fires when no safe split exists at all: a
    mis-routed claim, no unit left in the envelope set, or a split that could
    only be made by commanding one pack to charge while another discharges.
    Read `allocator.plan()` for each. main.py's refusal path and
    test_peakguard.py part 10h were re-aimed onto those rather than deleted -
    PR #40's reviewer proved that guard was unreachable and untested once, and
    removing its trigger without giving it a new one would rebuild the hole.
    """
    return allocate_record(target_w, bounds, weights=weights, live=live,
                           scope_errors=scope_errors,
                           site_priority=site_priority)[:2]


def allocate_record(target_w: float, bounds: dict, *, weights=None, live=None,
                    scope_errors=(), site_priority=None) -> tuple:
    """`allocate()` plus the record - `(shares, allocated, allocation)`."""
    rec = allocator.plan(target_w, bounds, weights=weights, live=live,
                         scope_errors=scope_errors,
                         site_priority=site_priority, site_key=ACTUATOR_SITE)
    return rec.shares, rec.ok, rec


def site_resolve(claims: list) -> Resolution:
    """STAGE 2. The SITE claims -> one grid target.

    ⚠️ THIS FUNCTION IS THE SHIPPED ARBITER, UNCHANGED. It is a separate name and
    the same code, deliberately: at N=1 nothing is filed on a unit, so `resolve()`
    hands it the whole claim list and it answers what it has always answered.
    Every check in test_arbiter.py, test_watchdog.py, test_validate.py and
    test_peakguard.py replays through here unedited, which is the proof that
    ARB-03 changed no behaviour on the live path.
    """
    sets = sorted((c for c in claims if c.kind == SET),
                  key=lambda c: c.priority, reverse=True)

    if sets:
        winner = sets[0]
    else:
        # No strategy asked for anything. That is not "carry on as before" - the
        # inverter holds its last command forever, so silence must mean zero.
        winner = Claim.set("failsafe", P_SAFETY, 0.0, "no claim was made")

    lo, hi = -inf, inf
    lo_src = hi_src = None
    for c in claims:
        if c.kind != LIMIT or c.priority < winner.priority:
            continue
        if c.lo > lo:
            lo, lo_src = c.lo, c
        if c.hi < hi:
            hi, hi_src = c.hi, c

    considered = [f"{c.source}:{c.kind}" for c in claims]

    if lo > hi:
        return Resolution(0.0, winner.source, bound_by=None,
                          contradiction=True, considered=considered,
                          winner_priority=winner.priority)

    target = max(lo, min(hi, winner.value))
    bound_by = None
    if target != winner.value:
        binder = lo_src if target == lo else hi_src
        if binder is not None:
            bound_by = f"{binder.source}({binder.reason})" if binder.reason else binder.source

    return Resolution(float(target), winner.source, bound_by, False, considered,
                      winner_priority=winner.priority)


def resolve(claims: list, *, units=None, weights=None, live=None) -> Resolution:
    """The three stages, in order, over one cycle's claims.

    `units` is the site's roster of inverters. Left None it is derived from the
    claims themselves, which is right for a caller that files a claim for every
    unit every cycle (main.py does - `supervised` is unconditional) and wrong for
    one that does not, so main.py passes the real roster rather than let a silent
    unit look like a smaller site.

    `weights` and `live` are ALLOC-01's two per-tick inputs to stage 3, and both
    are per-TICK on purpose. `weights` is the comfort choice - SoC, thermals,
    wear - and cannot destabilise the site whatever it says. `live` is the set
    of units whose master has reported recently enough to be commanded; a unit
    left out of it is DROPPED from the envelope set and the record says so,
    because the survivor carrying a dead unit's share silently is the failure
    SAFETY-30 is about. ⚠️ NEITHER MAY BE CACHED BY A CALLER. A unit's range
    moves for reasons unrelated to the allocator - a stale link, a pack at its
    floor, a device rating that failed to read - so an envelope read once at
    construction over-allocates into a unit that cannot take it.

    ⚠️ AND THE ARGUMENT HAS TO BE HONOURED, WHICH IS NOT THE SAME AS EXISTING.
    Ignore it - derive always - and a unit that filed nothing this cycle ceases
    to exist: a site of two reads as a site of one and stage 3 passes the whole
    grid target through to the inverter that spoke, which is ARCH-01's fault
    with its own evidence removed. That mutation survived the whole suite until
    test_arbiter.py part 7 graded it on a SILENT unit.

    ⚠️ THE PARTITION IS BY `actuator`, AND IT IS A PARTITION, NOT A FILTER. Every
    claim lands in exactly one stage. That is the whole structural argument: a
    unit's `hi` and the site's `lo` are never both terms of the same `lo > hi`
    test, so SAFETY-25 §0a's cross-inverter contradiction - 0 W inside the billed
    quarter - is not something to guard against, it is something that cannot be
    written down.
    """
    site_claims = [c for c in claims if c.actuator == ACTUATOR_SITE]
    if units is None:
        units = {c.actuator for c in claims if c.actuator != ACTUATOR_SITE}
    units = tuple(sorted(units))

    # STAGE 1 - each unit's own claims become that unit's interval.
    bounds = {u: pre_resolve([c for c in claims if c.actuator == u])
              for u in units}

    # STAGE 2 - the site claims become the grid target. At N=1 `site_claims` IS
    # `claims`, so this is the shipped arbiter on the shipped list.
    out = site_resolve(site_claims)

    # STAGE 3 - the seam, and since ALLOC-01 the allocator behind it.
    #
    # ⚠️ THE ORDER MATTERS: `scope_violations()` IS COMPUTED BEFORE THE SPLIT,
    # NOT AFTER, because stage 3 consumes it. A claim filed on an actuator its
    # source's scope does not permit means the number about to be divided may
    # itself be wrong, and dividing a number that may be wrong is worse than
    # not dividing it - `allocator.plan()` carries the full argument, including
    # why the mis-routed claim is NOT promoted into somebody's envelope. This
    # was the open question ARB-03 left, and it is answered here rather than
    # left as a log line nothing grades.
    out.scope_errors = scope_violations(claims)
    out.unit_bounds = bounds
    out.shares, out.allocated, out.allocation = allocate_record(
        out.target_w, bounds, weights=weights, live=live,
        scope_errors=out.scope_errors, site_priority=out.winner_priority)
    return out