Hold the store lock across find and write in correct and delete

[HUAN2022A]

Fixes #16

What was wrong

Store.correct had a TOCTOU race: it read the record via self.find(name)
while holding no lock, took the store lock only for the provenance append, and
then called self.write(), which acquires a second, independent lock to
persist what was by then a stale snapshot. A concurrent writer — another
correct, or any pass mutating the same memory — could land between the
find() and the final write(), and correct() would silently overwrite its
changes: lost edits, and lost provenance. delete() has the same shape — an
unlocked find, an in-memory invalidate, then a separately locked write —
so a delete racing a writer could clobber freshly appended provenance or
write stale content over another writer's update.

The fix

• correct and delete now take the store lock once and hold it across the
  whole read-modify-write: the find, every mutation, and the persist.
• The persist step goes through a new _write_locked helper that reuses the
  existing write machinery (validate, canonicalise dates, write, reproject)
  without re-acquiring the lock, so a caller that already holds the lock
  cannot double-lock or deadlock. The public write() still acquires the lock
  itself and delegates to the same helper, so external callers are unchanged.

Tests

tests/system/test_write_races.py spawns real OS processes (multiprocessing
with the spawn start method) against one shared memory:

• test_concurrent_corrects_keep_every_provenance runs four correctors, 12
  provenance excerpts each, and pins the accumulation invariant: every one of
  the 48 excerpts is present in the final record and
  len(provenance) == 48 — nothing dropped, nothing truncated.
• test_delete_racing_corrects_never_ends_active runs a deleter against the
  same four correctors and pins delete's legal terminal state: whatever the
  interleaving, the record never ends active — it is invalid with
  invalid_at set, and every worker exits cleanly.

Verification

Verification (Linux, Python 3.12, WSL, from this branch):

• uv run pytest -q: ............................................. [ 18%]
  ........................................................................ [ 36%]
  ........................................................................ [ 55%]
  ........................................................................ [ 73%]
  ........................................................................ [ 92%]
  ............................ [100%]
  390 passed in 47.12s
  w s l : : 纇Km0R l o c a l h o s t 鉔tM憂FO*g\曄P0R W S L0N A T !j_N剉 W S L N/e c l o c a l h o s t 鉔t0

• uv run ruff check .: All checks passed!
  w s l : : 纇Km0R l o c a l h o s t 鉔tM憂FO*g\曄P0R W S L0N A T !j_N剉 W S L N/e c l o c a l h o s t 鉔t0

• uv run mypy: Success: no issues found in 66 source files
  w s l : : 纇Km0R l o c a l h o s t 鉔tM憂FO*g\曄P0R W S L0N A T !j_N剉 W S L N/e c l o c a l h o s t 鉔t0