Three gaps in the #110544 guard, all reported in its review and reproduced:
- A writer reopened by _reopen_after_close_locked (teardown/worker race,
#94736) came back with no guard: the next stray close + foreign close
deleted its WAL again.
- _try_wal_checkpoint refreshed the guard outside self._lock; landing after
close() it pinned an OFD lock with no connection behind it, so a foreign
`PRAGMA journal_mode=DELETE` saw `database is locked` forever.
- Refcounts keyed on (fd, inode) treated a recycled fd number as a surviving
lock: A+B live, close A, C reuses A's fd, close B left C recorded as guarded
while a foreign EXCLUSIVE succeeded.
The guard now counts handles per inode, re-locks every matching descriptor on
each hold (OFD re-lock is idempotent), and unlocks on the last handle only;
the reopen path holds it; the checkpoint refresh runs under self._lock and
skips a closed handle. The macOS holder scan folds case so a case-only alias
of the sidecar path on APFS still matches.
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