library: import Traktor collection.nml (tracks, playlists, cues)

[vynulldev]

What & why

Adds a Traktor NML importer alongside the existing rekordbox XML / master.db / backup importers, so a Traktor user can bring their whole library into Vynull. First non-rekordbox library format. Traktor stores everything in one XML file (collection.nml), so this mirrors the rekordbox-XML importer and plugs into the same import pipeline.

Imports:

• Tracks: title, artist, album, genre, BPM, key, rating, year, track number, label, comment, play count, bitrate, file size. Key prefers the text INFO KEY (e.g. "Am") and falls back to mapping Traktor's MUSICAL_KEY VALUE (0 to 23) to standard notation; rating maps Traktor's 0 to 255 scale to 0 to 5 stars; bitrate converts bits/s to kbps.
• Playlists: the full nested folder + playlist tree, resolving each PRIMARYKEY back to a track via its VOLUME+DIR+FILE location key.
• Cues: hot cues (Traktor's 0 to 7 mapped to the 1 to 8 slot convention), memory cues, and loops from CUE_V2, reusing the existing import cue carrier so they land in the cue store with no new plumbing. The grid anchor (TYPE 4) and stray fade markers are dropped.

Paths: Traktor encodes the directory with "/:" separators, so the importer rebuilds the real path, prepends a Windows drive volume, and leaves the mac/Linux boot volume off. Files that do not resolve locally fall through to the existing missing-file flagging and the import dialog's path-remap field, exactly like a rekordbox import from another machine.

UI: the import dialog gets a Traktor source radio, the tracks/playlists/cues include options, and a collection.nml placeholder. Because the form now handles more than rekordbox, the source-specific copy is generalized ("Import your library" / "Import a library", "IMPORT LIBRARY").

Caveats: best-effort path reconstruction across platforms, and worth validating against a real collection.nml since the NML format drifts between Traktor versions. Traktor CUE_V2 carries no per-cue colour, so imported cues take the default.

Hardware testing

• Tested on: N/A for code paths: no change to the load, serve, or analysis code; imported tracks and cues flow to decks through the existing rekordbox serving path. That said, this feeds new data (Traktor-authored cues, BPM, paths) to a deck for the first time, so a sanity check on hardware (load a Traktor-imported track, confirm it plays and its hot cues render) is recommended before relying on it.

Checklist

• go build ./..., go vet ./..., and go test ./... pass
• gofmt -l . is clean
• New source files carry an SPDX header (GPL-3.0-or-later)
• Tested on real hardware (deck + firmware noted above), or this change doesn't affect deck behaviour
• I agree my contribution is licensed under the project's GPLv3