test(watcher): fix fs.watch flake by adding settle delay

[mschreib28]

Summary\n\nThree watcher tests wrote files immediately after watcher.start(), hitting a race with macOS FSEvents (and Linux inotify): there's a small but real latency between fs.watch() returning and the kernel actually delivering events. Under parallel test load (when the host CPU is busy running other test files in worker threads), that latency balloons and the file-change event is dropped before the watcher is fully registered. Result: the test waits 5s for a sync that never fires and times out.\n\n## Affected tests\n\n- debounced sync > should trigger sync after file change — most frequently affected\n- callbacks > should call onSyncComplete after successful sync\n- callbacks > should call onSyncError when sync throws\n- debounced sync > should debounce rapid changes — less affected (its 50ms-spaced loop incidentally settles, but explicit is better)\n- CodeGraph integration > should auto-sync when files change\n\n## What changed\n\nThe filtering tests in this file already had a 400ms settle delay with a comment ("Let watcher settle — fs.watch may fire residual events from beforeEach"). This PR:\n\n- Factors that out into a single letWatcherSettle() helper with a docstring explaining the FSEvents/inotify latency.\n- Applies it consistently to every test that writes immediately after start().\n- No production code change — the flake had no user impact, it was purely a test-order artifact under parallel load.\n\n## Verification\n\n- Pre-fix: ran npm test 8+ times across the session that produced PRs colbymchenry#100–colbymchenry#108. The fs.watch flake fired in roughly 1 of 3 runs under parallel load.\n- Post-fix: 3 consecutive npm test runs, 380/380 each, zero flakes.\n\n🤖 Generated with Claude Code\n

---

Copied from colbymchenry/codegraph#109