Address code-review findings on the metadata pipeline:
- finalize_known! now fsyncs the sidecar bytes before the rename and
fsyncs done/ after, so the "file in done/ implies sidecar present"
invariant holds across power loss, not just process crashes. The
docstring previously claimed an fsync the code never performed.
- run_exiftool's timeout escalates SIGTERM -> (2s grace) -> SIGKILL, so
an exiftool that ignores SIGTERM can't pin a worker forever on
wait(proc). Previously the timeout sent only SIGTERM.
- Add test/ (48 tests) covering the correctness-critical paths:
sanitize_filename, normalize_metadata, degraded build_metadata,
real exiftool extraction, finalize_known! end-to-end, recover_dir!.
