test(file-search): add performance harness

[JustYannicc]

What changed

• Added scripts/file-search-perf-harness.mjs, a deterministic temp-home benchmark for initial indexing, normal queries, path-like queries, watcher-style update batches, and delete/tombstone batches.
• Added scripts/test-file-search-perf-harness.mjs with generous regression thresholds and temp-home cleanup assertions.
• Added npm run perf:file-search as the discoverable local entrypoint.
• Added a narrow __fileSearchIndexPerfHarness export so the harness can reset/rebuild/apply watcher batches against a supplied temp home without relying on production timers or the real home directory.

Why

Optimization threads are expected to change file-search rebuilds, path queries, and delete/update handling. This gives those changes a repeatable local measurement target and a fast smoke test so regressions are visible before they reach review.

Compatibility impact

• No intended production search behavior change.
• The only runtime-source change is an additional exported perf-harness hook; normal indexing, watching, querying, and IPC paths are unchanged.
• The harness creates a synthetic OS-temp home, resolves symlinked temp paths, cleans up by default, and never indexes the real user home.

How tested

• node scripts/file-search-perf-harness.mjs --projects 12 --modules 4 --files-per-module 6 --query-runs 3 --updates 16 --deletes 16 --limit 10 --threshold-index-ms 15000 --threshold-normal-query-p95-ms 2000 --threshold-path-query-p95-ms 2000 --threshold-watch-update-ms 5000 --threshold-delete-ms 5000

File search perf harness
Indexed entries: 472
Initial indexing: 62.125ms
Normal queries: mean 0.740ms, p95 6.477ms, results 120
Path-like queries: mean 0.543ms, p95 1.145ms, results 90
Watcher-style updates: 0.454ms for 16 paths
Delete batch: 1.081ms for 16 paths
Post-update query: 0.215ms (10 results)
Deleted exact matches after tombstone: 0
Thresholds: passed

• node --test scripts/test-file-search-perf-harness.mjs
• node --test 'scripts/test-*.mjs'
• node --check scripts/file-search-perf-harness.mjs
• node --check scripts/test-file-search-perf-harness.mjs
• git diff --check
• ./node_modules/.bin/tsc -p tsconfig.main.json
• node scripts/check-i18n.mjs exits 0; it still reports existing Italian locale gaps for settings.ai.whisper.vocabulary and settings.extensions.appSearchScope.
• Codex LSP diagnostics reported no errors for src/main/file-search-index.ts, scripts/file-search-perf-harness.mjs, and scripts/test-file-search-perf-harness.mjs after dependencies were available.

[JustYannicc]

Superseded by the consolidated upstream PR #535: #535