SAFETY-28: gate the write path on the allocation record, not on key count

specs §1.5 makes "any configuration in which two writers can reach one
actuator" a non-goal. That non-goal was enforced by counting config keys:
validate.py's ACTUATOR_WRITE_KEYS = ("setpoint_entity",), consumed at rule 1,
with a comment stating the guarantee outright - "there is exactly one such
key, so a second writer is not expressible in the options schema at all".

That was true and it graded THE SHAPE OF THE OPTIONS SCHEMA, not the writes.
Add CFG-04's per-inverter config - a roster of inverters each with its own
setpoint_entity - and there are two real writers with len() of that tuple
still equal to 1. The check stays green while the property it protects is
gone. sim/inject.py's two_writers scenario asserted on the same tuple, so it
went green too. That is this repository's named failure mode: a check that
observes something other than what it claims to observe.

Demonstrated rather than argued. test_validate.py part 2 now constructs the
hazardous per-inverter config, asserts that the OLD proxy is still green on
it, and then asserts the new check refuses it. Against release/1.0's
validator the updated sim scenario reports: "validate_startup_config()
ACCEPTED a configuration in which 3 declarations reach the ONE actuator
number.goodwe_setpoint ... the old proxy reports 1 write key(s) on this same
config and is therefore GREEN on it".

THE WRITE PATH IS NOW GATED ON THE RECORD
=========================================
Controller.write_refusal() asks Allocation.authorises() whether the single
allocator placed exactly this number, for this unit, on this tick.
write_setpoint() refuses anything else. A second writer with arithmetic of
its own cannot satisfy that - its number appears in no record - so it has to
file a Claim and go through resolve(), which is the one allocator. "Exactly
one allocator per shared resource" stops being a convention somebody can
forget and becomes something the write path checks.

The refusal is a SUBSTITUTION, not a return: 0 W goes on the wire in place of
the number nothing stands behind. Returning would leave the inverter latched
on its last command with the ESP32 heartbeat still being fed - holding an
unauthorised setpoint quietly, which is the outcome the gate exists to stop.
AC 1 is "refused at the write path, not merely logged", and test_clamp.py
part 7a grades the device's writes, not the log.

The gate sits BELOW the non-finite coercion and ABOVE the clamp. Below,
because a nan is loss of input and rule 1 has already turned it into 0 W.
Above, because the question is who chose this number, and the answer is only
meaningful about the number that was asked for - gating the CLAMPED value
would refuse exactly the cycles SAFETY-03 exists for, since a clamped write
differs from the allocator's share by construction. Mutation M7 moves the
gate below the clamp and turns nine checks red, part 6c among them.

A STALE RECORD CANNOT BE REPLAYED
=================================
Every Allocation now carries the `tick` it was made for. It is handed IN by
the caller - plan(..., tick=) - rather than counted inside the allocator,
because test_alloc.py part 10 grades that the module holds no state at all,
and a serial number incremented in there would be the cached-across-ticks
thing the envelope rule forbids wearing a different hat.

Controller.alloc_tick advances ONCE PER RESOLUTION, not once per loop
iteration, and both halves are load-bearing:

  * a new allocation makes the previous record stale immediately, so a record
    whose envelopes have moved cannot authorise a write. That is the replay
    half (AC 2), graded by capturing a real record, running another real
    cycle, putting the old one back and observing refusal.
  * the "inputs missing, not yet stale" hold returns BEFORE the allocator
    runs, exactly so one dropped meter reading does not zero the inverter,
    and the heartbeat re-affirms self.target through this same gate while it
    does. A tick incremented per loop iteration would refuse that heartbeat
    and turn the hold into the outage it exists to prevent. Graded.

An UNSTAMPED record (tick None) authorises nothing: "I cannot say when I was
made" fails closed rather than reading as tick zero.

THE FAILSAFE CANNOT DEADLOCK BEHIND THE GATE, AND HERE IS WHY
=============================================================
write_setpoint() is also how layer 2 commands 0 W. cycle()'s two refusal
branches substitute a 0 W failsafe and write it like any other cycle;
shutdown() and the operator's stop button do the same. EVERY ONE OF THOSE
RUNS WHILE THE ALLOCATOR HAS REFUSED - ok is False and there are no shares -
so a gate that asked the allocator's permission to command zero would block
the safety write behind the very component whose failure the safety write
answers. A gate that can do that is worse than the hole it closes.

Resolved by authorising 0 W unconditionally, and on the VALUE rather than on
a failsafe=True flag: a flag is an assertion by the caller, which is exactly
what a second writer would pass; the value is a fact about what will be
written. The argument that this is safe and not merely convenient is that the
hazard §1.5 names is N-times loop gain from BROADCAST - handing every inverter
the same target so each supplies it in full (allocator.py's opening argument;
TEST-19 measured a permanent 4000 W p2p limit cycle at gain 1.0 with no guard
firing). At S = 0 the broadcast and the correct allocation COINCIDE: a_i = 1
for every unit gives every unit 0 W, which is exactly its share. There is no
over-command to make at zero. Said at the line, in write_refusal().

test_clamp.py part 7c drives it end to end through the real cycle() and the
real REFUSE_EMPTY branch: the allocator refuses every tick, the device still
sees 0 W, and the action log says ARBITER: REFUSED rather than WRITE REFUSED
so the two failures stay distinguishable to whoever has to debug them.

unmet_w IS STILL NOT ok
=======================
The gate reads ok and the unit's share and never unmet_w. allocator.py:179
says why: an allocation that could not place every watt is still safe to
write, and fusing them would let one saturated inverter park the whole site.
Graded on a record with 3800 W unmet and every share legal - mutation M5 adds
`or self.unmet_w` to the gate and that check goes red.

THE CONFIG CHECK, REPLACED RATHER THAN DELETED
==============================================
actuator_writers() counts WRITERS PER ACTUATOR instead of option keys. It
walks the options TREE rather than looking for a container called `inverters`,
because CFG-04 has not chosen that name yet and a census that guessed wrong
would go green while two writers existed - the same defect one step along.
Entity ids are folded to lower case so a second writer cannot be smuggled in
as Number.X versus number.x.

Two inverters with two entities are ACCEPTED - that is what the per-inverter
roster is for, and a validator that refuses the feature it guards is one
somebody switches off. Two declarations aimed at ONE entity are refused,
naming the actuator and every writer pointed at it. That includes the case an
author will meet first - keeping setpoint_entity as A's and listing A again -
because "they happen to be the same entity, so it is fine" is precisely the
reasoning that makes two writers invisible.

ACTUATOR_WRITE_KEYS survives ONLY as the enumeration a census needs, with its
guarantee comment deleted; nothing asserts on its length any more. Keeping the
name is also what stops admin/goodwe's current two_writers scenario failing on
an AttributeError between the two merges.

WHAT NEITHER CHECK OBSERVES, SAID OUT LOUD
==========================================
The census sees CONFIGURED writers. A second write path added in CODE - a
hass.set_number() call next to some new strategy - declares nothing and is
invisible to it, exactly as it was invisible to the tuple. That half is not
closed at startup and cannot be; it is closed at runtime by the record gate,
which does not care what the options look like or who called the method. The
two checks are deliberately different kinds, and only the runtime one survives
a schema this file has not seen yet.

EVIDENCE
========
* Full 18-file suite green on a purged tree (tools/mutation_run.sh).
* Verified by breaking it - seven mutations, each on a purged tree, each
  reverted and re-verified:
    M1 remove the gate                -> test_clamp red (4 checks)
    M2 make the zero exit an amnesty  -> test_clamp red (12 checks)
    M3 drop the tick comparison       -> test_clamp red (5 checks)
    M4 delete the census refusal      -> test_validate red (3 checks)
    M5 fuse unmet_w into ok           -> test_clamp red (1 check)
    M6 freeze the tick on the live path -> test_clamp red (2 checks)
    M7 move the gate below the clamp  -> test_clamp red (9 checks)
* AC 5: no existing check was weakened. The two edits to test_clamp.py are
  fixtures, not assertions - parts 5 and 6c call write_setpoint() directly
  with a number nothing allocated, which is now refused, so they say where
  their number came from. Every assertion in both parts is unchanged, and
  part 7f asserts that across part 3's sixty real cycles the gate refused
  zero writes.
* admin/goodwe's TEST-19 norig scenarios (planner_binding, two_master_plant,
  two_master_broadcast) still pass against this tree - they import the
  add-on's control law across the repo boundary.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>

goodwe_controller/app/validate.py

@@ -229,17 +229,84 @@ _LOG = logging.getLogger("goodwe.config")
 # a matching integer means a matching rule.
 FIRMWARE_INGEST_VETO_S = 30.0
 
-# ⚠️ The complete set of option keys through which this add-on can reach an
-# actuator. specs §1.5 makes "any configuration in which two writers can reach
-# one actuator" a non-goal, and the way that non-goal is ENFORCED here is by
-# construction: there is exactly one such key, so a second writer is not
-# expressible in the options schema at all. sim/inject.py's `two_writers`
-# scenario asserts on this tuple - if a second write path is ever added, that
-# scenario goes red and somebody has to decide how the two are arbitrated
-# BEFORE the config that allows both can ship.
+# ⚠️ THE OPTION KEYS UNDER WHICH A WRITER CAN BE DECLARED - AN ENUMERATION, AND
+# SAFETY-28 DEMOTED IT TO ONE. This tuple used to BE the guarantee. The comment
+# that stood here said specs §1.5 makes "any configuration in which two writers
+# can reach one actuator" a non-goal, and that the non-goal was ENFORCED by
+# construction: exactly one such key, so a second writer is not expressible in
+# the options schema at all.
+#
+# That was true, and it graded THE SHAPE OF THE OPTIONS SCHEMA rather than the
+# writes. Add CFG-04's per-inverter config - a list of inverters, each with its
+# own `setpoint_entity` - and there are two real writers with `len()` of this
+# tuple still equal to 1. The check stays green while the property it protects
+# is gone, which is this repository's named failure mode: a check that observes
+# something other than what it claims to observe.
+#
+# So the guarantee moved to `actuator_writers()` below, which counts WRITERS PER
+# ACTUATOR rather than keys, and finds them wherever in the options tree they
+# are declared. This tuple survives only because a census has to know which key
+# NAMES an actuator; it is not a bound on how many writers there can be, and
+# nothing should ever assert on its length again.
 ACTUATOR_WRITE_KEYS = ("setpoint_entity",)
 
 
+def _collect_writers(node, path: str, out: list) -> None:
+    """Walk the options tree, collecting `(where, entity)` for every writer.
+
+    ⚠️ STRUCTURAL, NOT NAME-BASED ON THE CONTAINER, and that is the point. A
+    census that looked for a key called `inverters` would be guessing the name
+    CFG-04 has not chosen yet, and a guess that is wrong goes GREEN while two
+    writers exist - the same defect one level along. This walks whatever shape
+    the options happen to have and reports every `setpoint_entity` in it,
+    however deeply nested and under whatever container.
+    """
+    if isinstance(node, dict):
+        for k, v in node.items():
+            here = f"{path}.{k}" if path else str(k)
+            if k in ACTUATOR_WRITE_KEYS:
+                entity = str(v or "").strip()
+                if entity:
+                    out.append((here, entity))
+                continue
+            _collect_writers(v, here, out)
+    elif isinstance(node, (list, tuple)):
+        for i, v in enumerate(node):
+            _collect_writers(v, f"{path}[{i}]", out)
+
+
+def actuator_writers(opts: dict) -> dict:
+    """actuator -> the configured writers that can reach it. INSTANCE-AWARE.
+
+    The replacement for `len(ACTUATOR_WRITE_KEYS) == 1`. It answers HOW MANY
+    THINGS CAN WRITE TO EACH ACTUATOR, which is the property §1.5 actually
+    names, and it keeps answering it when the schema grows a per-inverter list:
+    two inverters with two entities are two writers on two actuators and fine,
+    two declarations naming ONE entity are the non-goal and are refused.
+
+    The key is the entity id, lowercased - Home Assistant entity ids are
+    lowercase by construction, and folding case here means a config cannot
+    smuggle a second writer past the census on `Number.X` versus `number.x`.
+
+    ⚠️ SAY WHAT THIS STILL DOES NOT OBSERVE, because the argument that demoted
+    the tuple applies to its replacement too. This is a census of CONFIGURED
+    writers. A second write path added in CODE - a `hass.set_number()` call next
+    to some new strategy - declares nothing and is invisible here, exactly as it
+    was invisible to the tuple. That hole is not closed at startup and cannot
+    be: it is closed at runtime, by `Controller.write_refusal()`, which refuses
+    any value the single allocator did not place on this tick and does not care
+    what the options look like. The two checks are deliberately different
+    kinds - one grades the config, one grades the writes - and only the second
+    one survives a schema this file has not seen yet.
+    """
+    found: list = []
+    _collect_writers(opts, "", found)
+    census: dict = {}
+    for where, entity in found:
+        census.setdefault(entity.lower(), []).append(where)
+    return census
+
+
 class ConfigRefused(Exception):
     """The configuration is wrong. Not "something broke" - see main.py's handler.
 
@@ -265,9 +332,8 @@ def validate_startup_config(opts: dict, dev_min=None, dev_max=None) -> list[str]
     """
     problems: list[str] = []
 
-    # --- 1. an actuator to write to ------------------------------------------
-    writers = [k for k in ACTUATOR_WRITE_KEYS
-               if str(opts.get(k, "") or "").strip()]
+    # --- 1. an actuator to write to, and AT MOST ONE WRITER PER ACTUATOR -----
+    writers = actuator_writers(opts)
     if not writers:
         problems.append(
             "setpoint_entity is not set, so this add-on has no actuator: every "
@@ -275,6 +341,29 @@ def validate_startup_config(opts: dict, dev_min=None, dev_max=None) -> list[str]
             "ESP32 heartbeat that holds off the layer-1 failsafe never happens. "
             "It would run, look healthy, and move no energy at all. Set it to "
             "the ESPHome number entity that carries the setpoint.")
+    # SAFETY-28. The §1.5 non-goal, graded on the writers rather than on the
+    # count of option keys. Read `actuator_writers()` for why the old check went
+    # green on the very config this one is here to refuse.
+    #
+    # ⚠️ AND IT REFUSES A DUPLICATE DECLARATION EVEN WHEN BOTH NAME THE SAME
+    # ENTITY, which is the case an author of the per-inverter config will meet
+    # first: keep `setpoint_entity` as inverter A's and add A to the list again
+    # and there are two config sites aimed at one actuator, each with its own
+    # neighbouring options. "They happen to be the same entity, so it is fine"
+    # is precisely the reasoning that makes two writers invisible. Declare each
+    # actuator exactly once and the refusal does not fire.
+    for actuator, who in sorted(writers.items()):
+        if len(who) > 1:
+            problems.append(
+                f"{len(who)} configured writers can reach the one actuator "
+                f"{actuator}: {', '.join(who)}. specs §1.5 makes any "
+                "configuration in which two writers can reach one actuator a "
+                "non-goal - two independent setpoints on one inverter is the "
+                "arbitration problem this add-on refuses to have, and at "
+                "runtime it is a meter-emulation target being written twice a "
+                "second by two strategies that cannot see each other. Give "
+                "each actuator exactly one writer, or point the second one at "
+                "its own entity.")
 
     # --- 2. the configured ceiling against the device's own rating -----------
     max_w = _number(opts, "max_w", 2000.0)