feat(auto): browse and play local music folders in Android Auto [size:exception]

Phase 1: pick a device folder via SAF (persisted grant, no new
permission), browse its nested subfolders/tracks as a 5th Android
Auto root folder (hidden until configured), and play tracks through
the existing pipeline (EQ, art rotation, cold-start-safe source).
No metadata/sort/filter/shuffle yet -- filename is the title, generic
rotating art is the placeholder; deferred to a follow-up phase.

Adds a new pluriwave/file_actions native method (listAudioChildren)
and an onActivityResult override in MainActivity for the SAF folder
picker -- both static-review-only, no Android build available here.

lib/modelos/pista_local.dart

@@ -0,0 +1,46 @@
+/// Local file-system node returned by the native `listAudioChildren`
+/// channel call (Design "Interfaces / Contracts"): either a subfolder or an
+/// audio file, one SAF tree level deep. Pure DTO — no behavior, so no unit
+/// tests are warranted for it on its own (exercised indirectly through its
+/// consumers, e.g. `ConstructorArbolAuto.itemsLocales`).
+class NodoLocal {
+  const NodoLocal({
+    required this.documentId,
+    required this.nombre,
+    required this.esDirectorio,
+  });
+
+  /// Opaque SAF document id, unique within the picked tree. May itself
+  /// contain `:`/`/` (Design "Prefix stripped by length"), so callers must
+  /// never split/parse it — only wrap it verbatim in a media id.
+  final String documentId;
+
+  /// Raw on-device filename (or folder name), NOT yet title-stripped.
+  final String nombre;
+
+  /// Whether this node is a browsable subfolder (`true`) or a playable
+  /// audio file (`false`).
+  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.
+class PistaLocal {
+  const PistaLocal({
+    required this.documentId,
+    required this.titulo,
+    required this.contentUri,
+  });
+
+  /// Opaque SAF document id for this track.
+  final String documentId;
+
+  /// Display title, already computed by the caller (filename minus
+  /// extension, or a derived fallback — see `navegacion_auto.dart`).
+  final String titulo;
+
+  /// Playable `content://` URI resolved via
+  /// `FuenteMusicaLocalAuto.uriContenidoDePista`.
+  final String contentUri;
+}