fix(alarm): start the fade-in when pre-started audio is already playing

The ringing screen only confirmed the native-to-Flutter handoff from
its playback-state listener, but app.dart pre-starts the station
before pushing the screen, so `reproduciendo` could be emitted before
the listener subscribed and no further event ever arrived. That
branch only cancelled the fallback timer: the gated fade-in never
started and the native alarm player was never told to stop, so the
alarm blared at the alarm-stream volume with no 5%-to-target ramp.
Previously this was a timing race the stream usually lost; gating the
ramp on the confirmation made the failure deterministic.

The already-playing branch now confirms the handoff explicitly
(idempotent with the listener), and the ramp re-imposes its 5% start
volume immediately instead of waiting for the first periodic tick.
Adds the regression test mounting in the real pre-started path.

test/pantallas/pantalla_alarma_sonando_test.dart

@@ -229,6 +229,30 @@ void main() {
     });
   });
 
+  group('handoff con audio prearrancado ya reproduciendo (regresion)', () {
+    testWidgets('si la radio ya esta reproduciendo al montar, la confirmacion '
+        'del handoff y el fade-in arrancan igual', (tester) async {
+      // Real production path: app.dart pre-starts the station BEFORE the
+      // screen mounts, so `reproduciendo` is emitted before the state
+      // listener subscribes and no further state event ever arrives. The
+      // handoff confirmation (native stop) and the Dart ramp must not
+      // depend on catching that already-missed event.
+      final entorno = await _montarPantalla(tester);
+
+      expect(
+        entorno.android.detenidas,
+        contains('ring1'),
+        reason: 'el nativo debe recibir el stop del handoff aunque '
+            '"reproduciendo" haya llegado antes del mount',
+      );
+      expect(
+        entorno.audio.volumenesAplicados,
+        [0.05, 0.85],
+        reason: 'el fade-in debe arrancar tambien en el camino ya-sonando',
+      );
+    });
+  });
+
   group('restore de volumen de medios con dispose como unico llamador', () {
     testWidgets('desmontar la pantalla sin detener ni posponer restaura el '
         'volumen exactamente una vez', (tester) async {