The previous implementation used tag.WriteTo() which only emits the ID3
tag (~311 bytes), leaving audio data behind. Files were truncated to just
the tag on every rating write.

New strategy:
1. Parse the 10-byte ID3v2 header to find the audio start offset.
2. Encode the new tag into an in-memory buffer via WriteTo.
3. Write tag + original audio into a temp file.
4. Try atomic os.Rename (works when Winamp does not hold the file).
5. Fall back to direct O_WRONLY|O_TRUNC write (works while Winamp plays,
   because Winamp opens with FILE_SHARE_WRITE on Windows).

Tests cover: POPM<->stars mapping, id3v2AudioStart, full round-trip
(1-5 stars + unrate) with audio-integrity check, and error cases.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>