feat(alarm): make the ring immune to device media volume

The alarm's steady-state audio runs on the Flutter media-stream
player after the native handoff, so device volume 0 silenced it
entirely. The ring now forces STREAM_MUSIC to an audible reference:
Dart requests the override before pre-starting alarm audio (fallback
WAV included), Kotlin captures the current volume once and restores
it idempotently on every exit path (dismiss, snooze, dispose), with
a native best-effort backstop in service teardown.

The backstop is handoff-aware via PluriWaveAlarmService.flutterOwnsRing:
confirmFlutterAudio marks the handoff before triggering the native
stop, so the backstop cannot restore the volume mid-ring right as the
Flutter player takes over (that would re-silence the alarm at volume
0). The flag resets at every ring start; Flutter process death after
handoff remains a documented best-effort gap.

The alarm's perceived loudness keeps ramping 5% to the configured
volume through the player as before; normal radio playback and call
ducking never touch the override.

Work unit 2/3 of alarm-volume-ramp-restore (ring volume override).

test/servicios/servicio_alarmas_android_test.dart

@@ -106,4 +106,32 @@ void main() {
       );
     },
   );
+
+  test(
+    'forzarVolumenMediaParaAlarma invoca overrideMediaVolumeForRing con fraction',
+    () async {
+      final servicio = ServicioAlarmasAndroid(channel: channel);
+
+      await servicio.forzarVolumenMediaParaAlarma(1.0);
+
+      final llamada = llamadas.singleWhere(
+        (c) => c.method == 'overrideMediaVolumeForRing',
+      );
+      expect(llamada.arguments, {'fraction': 1.0});
+    },
+  );
+
+  test(
+    'restaurarVolumenMedia invoca restoreMediaVolume sin argumentos',
+    () async {
+      final servicio = ServicioAlarmasAndroid(channel: channel);
+
+      await servicio.restaurarVolumenMedia();
+
+      final llamada = llamadas.singleWhere(
+        (c) => c.method == 'restoreMediaVolume',
+      );
+      expect(llamada.arguments, <String, Object?>{});
+    },
+  );
 }