feat(auto): browsable Android Auto media tree with play-by-id [size:exception]

Expose PluriWave to Android Auto (projected) as a media app:

- Declare car media support (automotive_app_desc.xml + manifest meta-data)
  so Android Auto discovers the existing MediaBrowserService.
- New navegacion_auto.dart: ConstructorArbolAuto builds the browse tree
  (Favoritos / Todas las emisoras / Mis emisoras, 50-item cap, stable
  emisora:<id> media ids), reproducirPorMediaId routes a car tap to the
  existing playMediaItem pipeline, FuenteEmisorasAutoLocal serves the tree
  cold-start-safe (local favorites/custom stations before Flutter UI runs).
- PluriWaveAudioHandler overrides getChildren/getMediaItem/playFromMediaId
  as thin delegations; playback pipeline untouched.
- EstadoRadio pushes live station snapshots to the browse source and
  reconciles the selected station when playback starts from the car.
- Every playable item ships title + artUri; stations without logo fall
  back to a bundled default art (android.resource://).

Tests: 52/52 green (10 new navegacion_auto, 2 new estado_radio, plus
audio safety-net suites). Handler overrides and native XML are
static-review-only (no Android build env). Size exception approved for a
single reviewable commit.

android/app/src/main/AndroidManifest.xml

@@ -107,6 +107,11 @@
         <meta-data
             android:name="flutterEmbedding"
             android:value="2" />
+
+        <!-- Android Auto discovery (android-auto-media) -->
+        <meta-data
+            android:name="com.google.android.gms.car.application"
+            android:resource="@xml/automotive_app_desc" />
     </application>
     <queries>
         <intent>

android/app/src/main/res/xml/automotive_app_desc.xml

@@ -0,0 +1,3 @@
+<automotiveApp>
+    <uses name="media"/>
+</automotiveApp>