fix(music): stop sharing one folder's cover across unrelated tracks, and merge various-artists compilations into one album

Two real, confirmed bugs in a large flat music library:

- enrich_audio.py's sibling-cover fallback assumed one folder is one
  release. A large flat "chart ranking" folder mixing dozens of unrelated
  artists carried several distinct WMP AlbumArt-cache guids (one per
  original album a track was ripped from), and the fallback picked
  whichever one WMP had copied to Folder.jpg — attaching one unrelated
  release's cover to every other track in the folder. Now refuses to pick
  a cover at all once 2+ distinct guids show up, rather than guess.

- music-app.js's groupMusicEntries grouped by artist first, album second,
  so a various-artists compilation (many genuinely different per-track
  artists, one shared album tag, no album-artist tag at all — a real
  ~20-track soundtrack rip has exactly this shape) could never be
  recognized as one release: every track landed alone in its own artist's
  bucket and got folded into a singleton pile. Now detects an album key
  shared across 2+ distinct artist keys and merges those tracks into one
  compilation card under a "Various" heading instead.

Both verified against real, previously-affected files and live in the
browser: the shared wrong cover is gone, and the compilation renders as
one card with all its tracks in order.

0 files changed, 0 insertions, 0 deletions