feat(search): invalidate file index cache on git file changes

[beruro]

Summary

• invalidate file-path indexes from repository watcher events instead of short TTL rebuilds
• coalesce frontend invalidation bursts and clear pending work on listener teardown
• single-flight equivalent index builds and reject stale generations after invalidation
• preserve the existing 32 MiB per-repository and 64 MiB global cache bounds

Verification

• cargo test --manifest-path src-tauri/Cargo.toml -p search file:: (9 passed)
• cargo check --manifest-path src-tauri/Cargo.toml -p org2
• tsc --noEmit -p tsconfig.json
• focused Vitest suites for file index invalidation and Tauri file search (7 passed)
• git diff --check

Performance review

• watcher events perform only cheap invalidation; no scan or polling timer is introduced
• burst debounce is scoped per repository and cleared on listener teardown
• equivalent builds share one flight; invalidation generations reject stale completions
• cache remains bounded by entry count and byte budgets