Foundation for the provider migrations. cacheFirst<D, T> wraps the
drift.watch() + REST cold-cache fallback pattern: yields cached rows
when present, kicks off REST fetch + drift populate when empty +
online, yields empty when offline. REST failures swallow to empty so
callers can surface errors via toast.
adapters.dart adds CachedX → XRef extension methods + reverse
XRef.toDrift() companions for Artist/Album/Track/Playlist. Adapters
accept some loss of server-derived fields (coverUrl, streamUrl,
ownerUsername) — UI already handles empty values; cold-cache fallback
briefly shows the real values before drift takes over.
cache_first_test covers all 4 branches (non-empty, empty+online,
empty+offline, REST failure). adapters_test covers basic round-trips.
Both safe to run on CI runner — no drift open required.
Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>
bvandeusen
4eb9935f8e