v0.3.1

Persistence hardening and stability

- Index persistence is crash-safe: writes go to a temp file, fsync, then
  rename into place instead of truncating the destination in place.
- Fixed a heap-use-after-free: saving the index while a live-monitoring
  mutation landed on another thread could read the pool mid-reallocation.
- Serialized index format rejects trailing bytes past the declared payload.
- Selected result and keyboard focus no longer drop during a live-monitoring
  refresh of an unrelated file.
- Documented symlink-skip and cache-staleness policies (README); tombstone
  reclamation confirmed rebuild-only, recorded in docs/adr/0007.
- CI: benchmark delta table no longer always reports "nothing to compare
  against" (cache-key collision fix).

Full test suite green; clang-format and cppcheck clean.