fix(watch): exit when the observer emitter thread dies silently

[yzxcj797]

Summary

A watchdog emitter reader thread can crash under heavy create/delete churn — the inotify _wd_for_path KeyError when Maven Surefire forks churn target/surefire/, or the Windows ReadDirectoryChangesW variant (both documented in #811) — while the observer thread itself stays alive. The child process keeps passing the daemon's proc.poll()-based health check while delivering no filesystem events: daemon status stays green, built_at_sha freezes, and every subsequent edit/git pull is missed until a manual restart.

The watch loop now checks _observer_threads_alive(observer) — the observer thread and every emitter thread — once per second. On the first dead thread it logs critical and raises SystemExit(1), exiting the child so the daemon's existing restart path recovers watch instead of silently serving a stale graph forever.

This deliberately implements the issue's fix option 2 (exit so poll-based health checks catch it) rather than option 1 (per-subdirectory scheduling): it covers both the Linux and Windows thread variants, and the reproducer requires churn that a Windows-CI test cannot drive. A follow-up can layer schedule-time ignore filtering on top without conflicting.

Testing

• New TestWatchObserverLiveness (3 cases): alive observer + alive emitter → healthy; dead observer → not healthy; dead emitter with alive observer → not healthy (the exact silent-death shape from watch: observer thread silently crashes on Maven target/ dir churn (KeyError in watchdog inotify) — daemon looks alive but stops updating #811).
• pytest tests/test_graph.py tests/test_incremental.py — 178 passed (2 symlink-privilege tests deselected, the same pre-existing Windows-environment limitation noted in fix(incremental): reconcile reverted file content #860/fix(graph): purge orphan rows during reconciliation #861/fix(graph): survive malformed extra JSON in get_all_files #870).
• ruff check code_review_graph/incremental.py tests/test_incremental.py — all checks passed.

Verification boundary: the end-to-end kill (driving a real inotify race under Maven churn) requires the Linux/WSL environment from the report; the unit tests pin the liveness predicate and the exit decision it feeds.

Fixes #811

[github-actions]

code-review-graph review

Overall risk: 0.35 (LOW) — 11 changed function(s)/class(es), 0 affected flow(s), 4 test gap(s)

Risk-scored changes

Risk	Level	Symbol	Location	Tested
0.35	low	tests/test_incremental.py::TestWatchObserverLiveness._Emitter	tests/test_incremental.py:1823	no
0.35	low	tests/test_incremental.py::TestWatchObserverLiveness._Shim	tests/test_incremental.py:1830	no
0.30	low	tests/test_incremental.py::_Emitter.__init__	tests/test_incremental.py:1824	no
0.30	low	tests/test_incremental.py::_Emitter.is_alive	tests/test_incremental.py:1827	yes
0.25	low	code_review_graph/incremental.py::_observer_threads_alive	code_review_graph/incremental.py:1540	yes
0.25	low	tests/test_incremental.py::TestWatchObserverLiveness._fake_observer	tests/test_incremental.py:1822	yes
0.15	low	code_review_graph/incremental.py::watch	code_review_graph/incremental.py:1555	yes
0.15	low	tests/test_incremental.py::TestWatchObserverLiveness	tests/test_incremental.py:1818	no
0.15	low	tests/test_incremental.py::TestWatchObserverLiveness.test_alive_observer_with_alive_emitter_is_healthy	tests/test_incremental.py:1836	(test)
0.15	low	tests/test_incremental.py::TestWatchObserverLiveness.test_dead_observer_is_not_healthy	tests/test_incremental.py:1841	(test)

Test gaps

• tests/test_incremental.py::TestWatchObserverLiveness (tests/test_incremental.py:1818)
• tests/test_incremental.py::TestWatchObserverLiveness._Emitter (tests/test_incremental.py:1823)
• tests/test_incremental.py::_Emitter.__init__ (tests/test_incremental.py:1824)
• tests/test_incremental.py::TestWatchObserverLiveness._Shim (tests/test_incremental.py:1830)

Token savings: this graph-backed report used ~31,387 fewer tokens (~93%) than reading every changed file in full (estimated, chars/4 approximation).

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