Add exclusive/shared file locking to the auth cache read-modify-write
cycle to prevent data loss from concurrent writers. Uses a dedicated
.lock file so the lock survives NamedTempFile::persist replacing the
data file.

- Use std::fs::File::lock()/lock_shared() (stabilized in Rust 1.89)
- Separate lock file (.lock extension) per auth hash
- Shared lock on reads, exclusive lock on writes
- Lock acquisition failures are tolerated (best-effort cache)
- Atomic write via NamedTempFile::persist with fallback to direct write
- Add 9 unit tests covering save/load, corruption, clear, concurrent
  writes
- Update MSRV to 1.89 (required for File::lock API)
- Remove skip_serializing_if from Project/Domain for postcard
  compatibility
- Move tempfile from dev-dependencies to dependencies (used in prod
  code)