feat(flutter): home screen on per-item rendering (Slice C)

End-to-end pilot of the per-item architecture. Home now reads from
the new homeIndexProvider (drift-first over CachedHomeIndex with
/api/home/index discovery + SWR), then each tile is a small
ConsumerWidget watching its own albumTileProvider/artistTileProvider/
trackTileProvider. Tiles render a matched-dimension skeleton while
their entity is still hydrating, and swap in the real card once
drift emits the populated row.

Track hydration is wired up — /api/tracks/:id already existed so
the queue's case 'track' just calls api.getTrack(id) and persists.

The visible behavior:
* Cold visit: small /api/home/index round-trip (IDs only, ~10×
  smaller than /api/home), then sections appear shaped with
  skeleton tiles; each tile materializes as the hydration queue
  drains. No more "30s blank → everything pops in at once."
* Warm visit: drift index emits instantly, drift entity rows emit
  instantly, no network. Page paints fully in the first frame.
* Mid-state: scrolling through a partially-hydrated section sees
  real cards next to skeleton cards. Layout doesn't shift because
  skeletons match real-card dimensions exactly.

CachedHomeSnapshot (and the legacy homeProvider) stay in place but
unconsumed by Flutter — left in for now so revert is cheap if the
new path needs reworking. Cleanup follow-up in a later slice.

Old /api/home endpoint untouched, so the web client keeps working
unchanged.

flutter_client/lib/api/endpoints/library.dart

@@ -3,6 +3,7 @@ import 'package:dio/dio.dart';
 import '../../models/album.dart';
 import '../../models/artist.dart';
 import '../../models/home_data.dart';
+import '../../models/home_index.dart';
 import '../../models/track.dart';
 
 /// LibraryApi wraps the server's native /api/* library surface.
@@ -27,6 +28,24 @@ class LibraryApi {
     return HomeData.fromJson(r.data ?? const {});
   }
 
+  /// GET /api/home/index — per-item rendering variant. Returns just IDs
+  /// per section; client hydrates each tile via the per-entity
+  /// endpoints. ~10× smaller than /api/home on populated libraries so
+  /// the cold-visit round-trip is correspondingly short.
+  Future<HomeIndex> getHomeIndex() async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/home/index');
+    return HomeIndex.fromJson(r.data ?? const {});
+  }
+
+  /// GET /api/tracks/{id}. Returns the canonical TrackRef. Used by
+  /// the HydrationQueue to populate cached_tracks on a per-tile miss
+  /// — the existing endpoint already joins album + artist so the
+  /// response carries everything TrackRef needs.
+  Future<TrackRef> getTrack(String id) async {
+    final r = await _dio.get<Map<String, dynamic>>('/api/tracks/$id');
+    return TrackRef.fromJson(r.data ?? const {});
+  }
+
   /// GET /api/artists/{id}. Server returns ArtistDetail which embeds
   /// ArtistRef inline; ArtistRef.fromJson already reads only the fields
   /// it cares about, so passing the whole body is correct.