Add rekordbox XML generator and validator

Walks a music folder tree and emits a rekordbox-compatible DJ_PLAYLISTS XML
where each directory becomes a playlist, preserving nesting as folder nodes.

- reads ID3, Vorbis and MP4 metadata (title, artist, album, BPM, key, ...)
- percent-encoded file:// locations, safe for umlauts and brackets in paths
- strips control characters that would make the XML unparseable
- skips 0-byte downloads and macOS AppleDouble stubs by default
- optional ffprobe fallback for durations mutagen cannot read

validate_rekordbox_xml.py checks the result before import: well-formedness,
resolvable locations, unique TrackIDs and intact playlist references.

Co-Authored-By: Claude Opus 5 <noreply@anthropic.com>

.gitignore

@@ -0,0 +1,9 @@
+__pycache__/
+*.py[cod]
+*.egg-info/
+.venv/
+venv/
+*.xml
+!example.xml
+.DS_Store
+Thumbs.db