feat(flutter): per-item rendering infrastructure (Slice A)

Plumbing for the per-item rendering pass — no UI changes yet, just
the layers the home/playlist/liked migrations will sit on.

* CachedHomeIndex drift table (schema 5→6) — section/position →
  entity-id rows, populated by the upcoming /api/home/index endpoint.
* HydrationQueue (concurrency=4, in-flight dedup) — bounded request
  pump that takes (entityType, entityId) and persists the result to
  the right cached_<entity> table. Albums + artists wired today;
  tracks deferred until /api/tracks/:id exists.
* Per-entity tile providers (albumTileProvider, artistTileProvider,
  trackTileProvider as StreamProvider.family) — watch drift, enqueue
  hydration on miss, yield AsyncValue<EntityRef?>.
* Skeleton widgets (album/artist/track) matched to the real card
  dimensions with a 1.2s shimmer sweep using FabledSword tokens. No
  shimmer-package dep — single AnimationController per surface.

See docs/superpowers/specs/2026-05-13-per-item-rendering-design.md
for the full architecture rationale.

flutter_client/lib/cache/db.dart

@@ -142,6 +142,27 @@ class CachedHistorySnapshot extends Table {
   Set<Column> get primaryKey => {id};
 }
 
+/// Section→position→entity-id index for the home screen, populated
+/// from the per-item discovery endpoint `/api/home/index`. Each row
+/// pins one tile slot to an entity; the actual entity data lives in
+/// cached_albums / cached_artists / cached_tracks. The home screen
+/// reads this table to know the layout, then hydrates each tile
+/// against the entity tables (per-item rendering). Schema 6+.
+class CachedHomeIndex extends Table {
+  /// One of: 'recently_added_albums', 'rediscover_albums',
+  /// 'rediscover_artists', 'most_played_tracks', 'last_played_artists'.
+  /// Mirrors the keys /api/home and /api/home/index emit.
+  TextColumn get section => text()();
+  IntColumn get position => integer()();
+  /// One of: 'album', 'artist', 'track'. Used to dispatch hydration
+  /// to the right per-entity endpoint.
+  TextColumn get entityType => text()();
+  TextColumn get entityId => text()();
+  DateTimeColumn get fetchedAt => dateTime().withDefault(currentDateAndTime)();
+  @override
+  Set<Column> get primaryKey => {section, position};
+}
+
 /// Caller-scoped quarantine ("hidden") rows. Mirrors the wire shape of
 /// /api/quarantine/mine — fully denormalized (track + album + artist
 /// fields inline) because the Hidden tab renders straight from this row
@@ -179,12 +200,13 @@ enum CacheSource { manual, autoLiked, autoPlaylist, autoPrefetch, incidental }
   CachedHomeSnapshot,
   CachedHistorySnapshot,
   CachedQuarantineMine,
+  CachedHomeIndex,
 ])
 class AppDb extends _$AppDb {
   AppDb([QueryExecutor? e]) : super(e ?? driftDatabase(name: 'minstrel_cache'));
 
   @override
-  int get schemaVersion => 5;
+  int get schemaVersion => 6;
 
   @override
   MigrationStrategy get migration => MigrationStrategy(
@@ -218,6 +240,13 @@ class AppDb extends _$AppDb {
             // populates. Optimistic flag/unflag also writes here.
             await m.createTable(cachedQuarantineMine);
           }
+          if (from < 6) {
+            // Schema 6: cached_home_index for per-item rendering.
+            // Empty on upgrade; first /api/home/index fetch populates.
+            // cached_home_snapshot stays in place for now — older
+            // builds still read from it as a fallback path.
+            await m.createTable(cachedHomeIndex);
+          }
         },
       );
 }