fix(node): stop inventing a fake artist from the shared root's name

Music grouping was measured against a real ~5700-file library and came
back worse than a plain file listing. Root cause: the artist/album
ancestor walk always climbed exactly two levels (parent = album,
grandparent = artist) with no idea where the group's own shared root
was. Any file in a flat top-level folder — common here: bare
`Artist/track.mp3`, no album subfolder at all — had its "grandparent"
resolve to the root directory's own name, so the artist got replaced
by the share's name. Measured: 289 of 5664 tracks across 41 real,
unrelated artists (Ben Harper, Dire Straits, Jimi Hendrix, Janis
Joplin, ...) collapsed into one fake artist this way — the single
biggest bucket in the whole library, ahead of every real one.

- `_artist_album_from_ancestors` now takes the entry's own root
  boundary (daemon.py resolves it via `RootSet.split`) and refuses to
  read it as a name. A file sitting in a top-level folder — genuinely
  ambiguous, artist or a standalone album/compilation — is handled by
  `_split_top_level_folder`: split on "Artist - Album" when the
  (cleaned) folder name has that shape, otherwise the whole name
  becomes the artist alone, the more common real case here.
- `_clean_tag` treats known tagger placeholders ("No Artist", a French
  tool's "Nouvel artiste (334)") as absent rather than a real value —
  they were just as truthy as a real name and were locking out the
  fallback that would have done better. "Various Artists" is kept, a
  real compilation credit rather than a placeholder.
- A `title` tag that's the bare filename copied verbatim (track number
  included — found live on a whole CD-single) is stripped through the
  same prefix rule the filename parser already used
  (`title_parse.strip_track_prefix`), since a tag normally wins over
  the parsed title.
- Cover art: only 11% of a 400-file sample had embedded art (expected
  for this era of rip), but 267 loose cover images sit beside the
  tracks across the library (Windows Media Player's `Folder.jpg`/
  `AlbumArt_{guid}_*.jpg`, manual `cover.jpg`) and were never looked
  at. `_find_sibling_cover` checks the track's own folder before
  giving up — measured coverage 11% -> 26% on the same library, zero
  network calls.

`_enriched_attempted` is in-memory and resets on restart, so a node
restart is enough to re-run enrichment over an already-scanned library
with the fixed logic — no rescan flag, no cache to clear by hand.

22 tests in test_enrich_audio.py (11 new), including the exact
regression case end to end through the real pool. Full suite: 1129
passed, no regressions.

Co-Authored-By: Claude Sonnet 5 <noreply@anthropic.com>
Claude-Session: https://claude.ai/code/session_01KBi7ALLGfwcjBXt57yNMcy

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