feat(xml): export device libraries as Rekordbox XML

[snkk2x-collab]

Addresses #127.

Summary

• add document_from_device_export to build a DJ_PLAYLISTS document from PIONEER/rekordbox/export.pdb
• export collection metadata, file://localhost locations, TrackID-keyed playlists, beatgrid TEMPO entries, and cue POSITION_MARK entries from matching ANLZ files
• add rekordcrate export-xml <EXPORT_PATH> [OUTPUT_FILE] for writing the generated XML
• add a fixture-based integration test over data/complete_export/demo_tracks

Validation

• rustfmt src/pdb/mod.rs src/xml.rs src/main.rs tests/test_xml_export.rs
• cargo test

[snkk2x-collab]

Superseded by #254, which contains the same fix as a clean single conventional commit after local cargo test validation.