fix(auth): guard every lock removal, not just release, and stop promising expiry (#2082)

Copilot review round 5. Both comments upheld; the first is a real hole in
round 4's own fix.

**The ownership guard was in the wrong place, and overclaimed.**

Round 4 checked ownership and then called `release()`. Two problems:

- Check-then-act. The `stat` and the `rmdir` were separate async steps, so a
  waiter could replace the directory in between and we deleted the winner's
  fresh lock anyway — the very thing the check was added to prevent.
- Skipping `release()` on a mismatch left proper-lockfile's record registered
  in its `locks` map, and its `signal-exit` handler `rmdirSync`s every
  registered lock with **no ownership check of its own**. An exit during that
  window deleted the winner too.

The guard now lives in `options.fs`, which is the single seam *both* removal
paths route through — `removeLock` on release, and the exit handler. It
refuses to remove a directory whose inode and birth time are not the ones
recorded at acquire, and reports the refusal as success so the library's
bookkeeping forgets the lock either way (leaving it registered is what hands
the exit handler a record pointing at the winner's directory).

The check is a `statSync` immediately followed by an `rmdirSync`, with no
`await` between them, so nothing in this process can interleave. **It still
does not close the race** — it is check-then-act across processes, which needs
the compare-and-swap Node does not expose — and saying it "closes the
destructive half outright" was wrong. file-lock.ts, README.md,
specification/v2_servers_file.md and AGENTS.md now all say it narrows the
window rather than closing it. AGENTS.md carries the negative instruction
alongside round 4's, since this is the third round spent on an overclaim.

**"It expires on its own after 10s" is false for the ENOTEMPTY case** the new
test exercises. Stale takeover reclaims through the same `rmdir`, which also
cannot remove a non-empty directory — so nothing clears it, and every later
save fails ELOCKED against it. `releaseAdvice` now branches: by-hand removal
for ENOTEMPTY, expiry for everything else.

Tests: a real child process demonstrates the exit handler removing a
replacement lock (with an **empty** directory — a non-empty one makes
`rmdirSync` fail ENOTEMPTY, which is how the first draft of this test passed
for the wrong reason). The release-failure test now asserts the message says
"by hand" and does not promise expiry. The shim's `rmdirSync` arm carries a
justified `v8 ignore`: it is reachable only from a real process exit, and its
logic is `removeIfMine`, covered through the async arm.

Co-Authored-By: Claude Opus 5 (1M context) <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01JTHVxSu8AUgHRLvo1ntZ8H
Signed-off-by: cliffhall <cliff@futurescale.com>

README.md

@@ -506,7 +506,7 @@ The Inspector writes the file `0600` and re-tightens it at startup if something
 
 Two running Inspectors are therefore genuinely serialized. What a lock file cannot make single-winner is the *takeover of a lock whose holder died* — that needs a compare-and-swap on a directory entry (`renameat2`) which Node does not expose, and it is what an earlier hand-rolled attempt failed three review rounds on. `proper-lockfile` does not close that race either. The window opens only after a holder dies without releasing.
 
-The Inspector adds one thing on top: before releasing, it checks the lock directory is still the one it created (by inode and birth time, which survive the library's own refresh but not a delete-and-recreate). That matters mostly because the library's release is an unconditional `rmdir` — so a holder whose lock had been replaced would otherwise delete the *winner's* lock on the way out, turning one compromised writer into two unprotected ones. It also surfaces the takeover as a warning. Treat that warning as **best-effort**, not a guarantee: it rests on filesystem metadata that not every filesystem reports.
+The Inspector adds one thing on top: every lock-directory removal the library makes on its behalf — on release, and from its exit handler — is guarded by a check that the directory is still the one it created (by inode and birth time, which survive the library's own refresh but not a delete-and-recreate). That matters because those removals are otherwise unconditional, so a holder whose lock had been replaced would delete the *winner's* lock on the way out, turning one compromised writer into two unprotected ones. It also surfaces the takeover as a warning. Treat all of this as **best-effort**: the guard is still a check followed by an act, so it makes the destructive case rare rather than impossible, and it rests on filesystem metadata that not every filesystem reports.
 
 Which is why, underneath the lock, each mutation still reads the file, applies its change, writes, then reads back and compares the whole map; if something wrote in between it re-applies onto what was left and retries, failing loudly after five lost rounds rather than returning as though the value were saved. That check is what still catches a clobber inside that window — and it covers what no lock can, since a lock only orders the writers that *take* it: an editor, a restored backup, or an Inspector older than this release.