Adds an embedded-art layer to the album cover chain between the
sidecar lookup and the external provider chain. Reads ID3v2 APIC,
FLAC METADATA_BLOCK_PICTURE, and MP4 covr atoms via the dhowden/tag
library (already a dependency for MBID extraction). Whatever Picard
or Lidarr embedded into the file at tag time becomes available
without any network call.
Diagnostic context: file managers (GNOME Files, Dolphin, etc.)
generate audio-file thumbnails by reading exactly this picture
block. An album that "has no art" in Minstrel but renders a
thumbnail in the file manager is the smoking-gun signal that
embedded art is present and we were ignoring it. Adds 'embedded'
to the chain so those albums get covered without any external
provider hit.
Chain after this change:
sidecar → embedded → MBCAA → TheAudioDB → Deezer → Last.fm → none
Embedded beats every external provider for accuracy because it's
the canonical art the operator/Picard chose at tag time, not a
guess from a remote catalog.
Refactors the post-fetch write logic into Enricher.writeAlbumArt
so the embedded layer and the provider chain share the same
sidecar-then-managed-cache fallback semantics. Behavior identical
to before for the provider path.
