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.

lib/pantallas/pantalla_alarma_sonando.dart

@@ -88,8 +88,15 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
     _fallbackTimer = Timer(const Duration(seconds: 12), () {
       if (mounted) _iniciarFallback();
     });
+    // Pre-started audio can reach `reproduciendo` BEFORE the listener above
+    // subscribes (app.dart starts the station before pushing this screen),
+    // in which case no further state event ever arrives. The handoff
+    // confirmation and the fade-in must not depend on catching that
+    // already-missed event, so this branch confirms explicitly too
+    // (idempotent — the listener firing as well is harmless).
     if (widget.audioPrearrancado && radio.audio.estaSonando) {
       _fallbackTimer?.cancel();
+      await _confirmarAudioFlutterListo();
     }
   }
 
@@ -111,6 +118,10 @@ class _PantallaAlarmaSonandoState extends State<PantallaAlarmaSonando> {
       unawaited(_aplicarVolumenGlobal(volumenObjetivo));
       return;
     }
+    // Re-impose the ramp's start volume immediately: the first periodic tick
+    // only lands after _fadeStep, and by now the player may have been
+    // recreated or re-leveled since the pre-start set it to 5%.
+    unawaited(_aplicarVolumenGlobal(inicio));
     final duracionTotalMs = segundosFade * 1000;
     final pasos = (duracionTotalMs / _fadeStep.inMilliseconds).ceil();
     var pasoActual = 0;