feat(auto): real metadata, quality sort and name buckets for local music [size:exception]

Local tracks now show embedded title/artist/album art (via native
MediaMetadataRetriever, cached through the existing FileProvider)
instead of the raw filename, falling back gracefully when a file
has no usable tags. Adds two navigable entry points per folder: sort
by audio quality (bitrate, capped at 150 tracks per folder to bound
worst-case latency) and alphabetical name buckets -- the closest
realistic form of "filtering" given Android Auto has no text-search
UI in this integration.

Metadata resolves only for the page actually being browsed (same
slice-cheap-then-map discipline as the paging change), backed by a
flat 256-entry LRU session cache that survives across pages. No new
permission, no new pub dependency, no l10n changes (car-tree labels
stay hardcoded Spanish, matching every existing label in the tree).

lib/modelos/pista_local.dart

@@ -23,14 +23,19 @@ class NodoLocal {
   final bool esDirectorio;
 }
 
-/// Minimal playable local track (Design "Phase 1 minimal shape" — no
-/// artist/album/duration metadata per spec's "Not in this delta"). Pure
-/// DTO — no behavior, so no unit tests are warranted for it on its own.
+/// Playable local track (Phase 2 extends the Phase 1 minimal shape with
+/// resolved embedded metadata — Design "Data Flow"). Pure DTO — no
+/// behavior, so no unit tests are warranted for it on its own beyond
+/// construction (`pista_local_test.dart`).
 class PistaLocal {
   const PistaLocal({
     required this.documentId,
     required this.titulo,
     required this.contentUri,
+    this.artista,
+    this.embeddedArtUri,
+    this.bitrate,
+    this.sampleRate,
   });
 
   /// Opaque SAF document id for this track.
@@ -43,4 +48,54 @@ class PistaLocal {
   /// Playable `content://` URI resolved via
   /// `FuenteMusicaLocalAuto.uriContenidoDePista`.
   final String contentUri;
+
+  /// Resolved embedded artist metadata, when available (Design "Interfaces
+  /// / Contracts" — `readAudioMetadataBatch`). `null` when unresolved or
+  /// unavailable.
+  final String? artista;
+
+  /// Resolved embedded-art `content://` URI served via the app's
+  /// `FileProvider` cache (Design ADR-1). `null` when there is no embedded
+  /// picture or it could not be resolved.
+  final String? embeddedArtUri;
+
+  /// Resolved bitrate in bits per second, when available.
+  final int? bitrate;
+
+  /// Resolved sample rate in Hz, only available on API 31+ (Design ADR-5).
+  /// `null` on older API levels or when unresolved.
+  final int? sampleRate;
+}
+
+/// Resolved embedded metadata for a single local track (Design "Interfaces
+/// / Contracts"): one entry per requested `documentId`, all fields
+/// individually nullable (per-field parse failures degrade gracefully
+/// instead of dropping the whole entry). Pure DTO — no behavior, so no
+/// unit tests are warranted for it on its own beyond construction
+/// (`pista_local_test.dart`).
+class MetadatosPista {
+  const MetadatosPista({
+    this.titulo,
+    this.artista,
+    this.artUri,
+    this.bitrate,
+    this.sampleRate,
+  });
+
+  /// Resolved embedded title, or `null` when absent/unparseable.
+  final String? titulo;
+
+  /// Resolved embedded artist, or `null` when absent/unparseable.
+  final String? artista;
+
+  /// Resolved embedded-art `content://` URI (Design ADR-1), or `null` when
+  /// there is no embedded picture or it could not be resolved/served.
+  final String? artUri;
+
+  /// Resolved bitrate in bits per second, or `null` when unknown.
+  final int? bitrate;
+
+  /// Resolved sample rate in Hz (API 31+ only, Design ADR-5), or `null`
+  /// when unknown/unsupported on this API level.
+  final int? sampleRate;
 }