Support multi-value TXXX/WXXX frames in ID3v2

[Rouzax]

Summary

• Add null-separated multi-value support to the TXXX/WXXX handler in id3.c
• Returns arrayref when multiple values present, scalar when single (no behavior change for existing files)
• Mirrors the existing STRINGLIST handler pattern for standard text frames (TPE1, TALB, etc.)
• Enables plural tags like TXXX:ALBUMARTISTS (written by Picard) to reach LMS as arrays

Context

Part of LMS-Community/slimserver#1555 (display_artist support). Vorbis/FLAC already deliver multi-value tags correctly; this closes the MP3 gap.

Forum discussion (post #133 in the thread): https://forums.lyrion.org/forum/user-forums/ripping-encoding-transcoding-tagging/1816764-lms-metadata-scaan-overrides-local-tags

Test plan

• Four new MP3 test fixtures covering: 2 values, 3 values, empty slot, single value
• 12 new test cases added to mp3.t
• All 413 tests pass (401 existing + 12 new)
• Single-value TXXX regression: existing tests at lines 329, 525, 933 still pass as scalars

[michaelherger]

Please note that we won't have new binaries for all possible platforms "automatically". I tend to only update recent, popular platforms by default. But I understand the change doesn't break existing installations, but rather adds a missing "feature" (support for multiple values). Therefore for users of outdated versions of this library nothing will change - neither good or bad.

[Rouzax]

Thanks for the note on platform coverage. That's completely fine.

The change is indeed additive only. Installations running an older Audio-Scan will keep getting a single scalar for TXXX frames, exactly as today. Nothing breaks for them.

On the slimserver side, the code that consumes these tags already needs to handle both scalars and arrayrefs anyway (some tag types already return arrays). So the scanner will work correctly regardless of which Audio-Scan version is installed: arrayref means multi-value support is available, scalar means it falls back to current behavior.