FCST-06: "cannot persist" becomes one question, asked once, about the outcome

price_history.py's docstring states its rule unconditionally - "A store that
cannot persist now records nothing, and both of its answers say the same
thing" - and FCST-04 enforced it against `not self.ok` only, which is one
SYMPTOM: the durable volume is missing. `ok` is about the DIRECTORY, and a
read-only remount and a full disk both leave it True. Reproduced on 5059e28
against two real volumes, a /data bind-mounted read-only and a full tmpfs:

    last_error = 'could not write ([Errno 30] Read-only file system)'
    words      = '2026-01-14 (24 points over 24 h); 2026-01-15 (24 points
                  over 24 h) [could not write (...)]'
    yesterday()= series, 24 points
    ON DISK    = {}

`words` and `yesterday()` were describing two different stores, and the 24
points were gone on the next restart with nothing saying so - the
silently-forgetting cache that rule exists to refuse, reached by the condition
it did not name. No shipped path reaches it: /data is the Supervisor's durable
volume and this module is its only writer. The defect is that a guarantee the
module states about itself did not hold in one condition.

Route taken: `_remember()` refuses to retain what it could not write, and the
docstring's claim stands as written. It is now ONE derived predicate rather
than two places testing two symptoms. "Can this store persist?" cannot be
answered BEFORE the attempt - a directory that existed and was writable a
moment ago is not a write that completed - so it is answered after it, once,
by the only thing holding the evidence: `_save()` returns whether the file now
holds what memory holds. No volume is one False, an OSError is another.
`_remember()` keeps only what that one answer says was kept, and the
`not self.ok` test in `_remember()` is GONE rather than joined by a second one.
FCST-04's M4 and FCST-05's HIGH were the same shape, so a third cause now needs
no third branch.

What is undone is the MERGE, not the store. The state restored is the last one
that IS on disk, so a site whose volume fills at 14:00 goes on answering the
yesterday it wrote at 13:30 - that record survives a restart, so answering it
is not a promise the store cannot keep. It stops adding to it. The snapshot is
copy.deepcopy: a one-level copy shares the day record the merge rebinds
`points` on, and measured with one, a store holding 12 persisted points
answered 24 after a failed merge.

Not fatal, deliberately: §6 and this module's opening - a broken cache must not
be a broken controller, and an add-on that will not start because /data filled
up is a worse defect than this one. `_save()` reports and returns; the shipped
resolve_tariff() path still gives a provider plan with no §11.4 degradation.
What changed is that the failure is now acted on rather than only logged.

test_price_history.py section 5d drives it on two REAL failures with different
syscalls and different errnos - a directory where the file goes (os.replace,
EISDIR) and a volume moved out from under an open store (mkstemp, ENOENT) -
because the rule is enforced on the outcome and one cause would grade a
coincidence. DOCS.md's "where there is no /data" sentence is widened to the
condition the code now holds to.

MUTATION.md gains what M4 exposed: a mutation whose red is a TRACEBACK reports
however many checks ran before it and grades none after it. M4 first printed
1 FAIL; the true figure is 3. An f-string operand read `len(read.points)`, and
`.points` on a no_data raises `Absence` by design - why()'s rule arriving in
what a check PRINTS rather than in what it asserts. `npts()` is now beside
why(), and a red count is a lower bound until the run is seen to reach the end
of its section.

6 mutations, purged tree, every one RED on named checks, no survivors:
M1 self._days = before -> dict(self._days)          +10 B  9 red
M2 _save() OSError handler returns True             -1 B   9 red
M3 _save() returns True on `not self.ok`            -1 B   3 red
M4 self._days = before -> self._days = {}           -4 B   3 red
M5 copy.deepcopy -> dict                            -9 B   3 red
M6 `if not self.ok: return` back in _remember()     +43 B  1 red

All 29 test files green. Nothing deployed; no option added; config.yaml and
app/ behaviour on a healthy /data untouched. No version number picked - that
is the open decision the Unreleased block names.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01BwRLw7eKYj2byVdwB6kbjU

goodwe_controller/CHANGELOG.md

@@ -14,6 +14,63 @@ if it has, it needs 0.10.0. Whoever deploys decides.
 |---|---|
 | _this branch_ | FCST-04 - the carry-forward fill reaches the plan, and the day it carries survives a restart |
 | _this branch_ | FCST-05 - one byte stops stopping the add-on, and the write path gets the read path's standard |
+| _this branch_ | FCST-06 - "cannot persist" stops meaning "has no /data" and starts meaning what it says |
+
+### FCST-06 - the second way to be a store that cannot persist
+
+**A price cache that could not write went on answering a yesterday the file did
+not contain.** `price_history.py`'s docstring states its rule unconditionally -
+*"A store that cannot persist now records nothing, and both of its answers say
+the same thing"* - and FCST-04 enforced it against `not self.ok`, which is one
+**symptom**: the durable volume is missing. `ok` is about the DIRECTORY, and a
+read-only remount and a full disk both leave it `True`. Reproduced on `5059e28`
+against two real volumes - a `/data` bind-mounted read-only and a full tmpfs:
+
+| | with a `/data` that cannot be written |
+|---|---|
+| `last_error` | `could not write ([Errno 30] Read-only file system)` |
+| `words` | `2026-01-14 (24 points over 24 h); 2026-01-15 (24 points over 24 h) [could not write (...)]` |
+| `yesterday()` | **series, 24 points** |
+| on disk | `{}` |
+
+`words` and `yesterday()` were describing two different stores, and the 24
+points were gone on the next restart with nothing saying so - the
+silently-forgetting cache that rule exists to refuse, reached by the condition
+it did not name. **No shipped path reaches it**: `/data` is the Supervisor's
+durable volume and this module is its only writer. The defect is that a
+guarantee the module states about itself did not hold in one condition.
+
+**"Cannot persist" is now ONE derived predicate, asked once, about the
+outcome.** It cannot be answered *before* the attempt - a directory that existed
+and was writable a moment ago is not a write that completed - so `_save()`
+returns whether the file now holds what memory holds: `False` with no durable
+volume, `False` on an `OSError` from the write, `True` otherwise. `_remember()`
+keeps only what that one answer says was kept, and **the `not self.ok` test in
+`_remember()` is gone** rather than joined by a second one. FCST-04's M4 and
+FCST-05's HIGH were the same shape - a rule enforced on the conditions somebody
+thought of - so a third cause now needs no third branch.
+
+**What is undone is the MERGE, not the store.** The state restored is the last
+one that *is on disk*, so a site whose volume fills at 14:00 goes on answering
+the yesterday it wrote at 13:30 - that record survives a restart, so answering
+it is not a promise the store cannot keep. It simply stops adding to it. The
+snapshot is a `copy.deepcopy`: a one-level copy shares the day record the merge
+rebinds `points` on, and measured with one, a store holding 12 persisted points
+answered **24** after a failed merge.
+
+⚠️ **A failed write is still not fatal, and that was a constraint rather than an
+oversight.** §6 and this module's opening: a broken cache must not be a broken
+controller. An add-on that will not start because `/data` filled up is a worse
+defect than the one being fixed, so `_save()` reports and returns - the failure
+stays in the log, in `last_error`, in `words` and in `yesterday()`'s reason -
+and the shipped `resolve_tariff()` path still returns a provider plan with no
+§11.4 degradation. What changed is that it is now *acted on*.
+
+`test_price_history.py` section 5d drives it on two real failures with different
+syscalls and different errnos - a directory where the file goes (`os.replace`,
+EISDIR) and a volume that moves out from under an open store (`mkstemp`,
+ENOENT) - because the rule is enforced on the outcome and one cause would grade
+a coincidence. See `DECISIONS/FCST-06-cannot-persist-is-one-question.md`.
 
 ### FCST-05 - what FCST-04's own contract still did not hold