bogem/id3v2 Save() renames a temp file over the original, which Windows
denies when Winamp holds the file open (no FILE_SHARE_DELETE). Fix:
- Use tag.WriteTo(&buf) to encode tag+audio into memory while the read
  handle is still open
- Close the read handle
- Reopen the original file with O_WRONLY|O_TRUNC and write the buffer

Winamp opens MP3s with FILE_SHARE_WRITE so the in-place overwrite succeeds.

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