refactor(alarm): make the ringing screen pure UI over a reduced native port

PantallaAlarmaSonando no longer owns any audio orchestration (fallback
player, dB ramp, native handoff confirm, media-volume override/restore):
it only calls EstadoAlarmas.finalizarEjecucion/posponerAlarma from
Stop/Snooze/back, keeping the single-exit guard, PopScope back=Stop and
dismiss semantics intact. The status line now reads directly from the
alarm's static config (station name or a neutral label) instead of a
live playback/handoff state.

PuertoAlarmasAndroid drops confirmarAudioFlutter,
forzarVolumenMediaParaAlarma and restaurarVolumenMedia, and app.dart no
longer pre-starts a station before pushing the ring screen. This is the
Dart half of moving to a single native ring-audio owner (WU1 of 2); the
Kotlin service rebuild lands next and keeps this intermediate state
shippable with no double audio.

test/helpers/fakes_alarmas.dart

@@ -17,31 +17,11 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
   bool ignoraOptimizacionBateria = true;
   int solicitudesExencionBateria = 0;
 
-  /// Records each [forzarVolumenMediaParaAlarma] call (Slice 2: ring-scoped
-  /// device-volume override).
-  final volumenForzado = <double>[];
-
-  /// Counts [restaurarVolumenMedia] calls (Slice 2). A plain counter, not a
-  /// list: widget tests only need to assert how many times restore ran.
-  int volumenRestaurado = 0;
-
   /// Test-only failure switch (Design D7): when true, [programar] throws
   /// instead of scheduling, enabling failure-path coverage that the fake
   /// could not otherwise produce.
   bool fallaProgramar = false;
 
-  /// Test-only gate (Slice 3: fade-in dedup at handoff). When set,
-  /// [confirmarAudioFlutter] suspends on this completer before resolving,
-  /// letting a test observe the pre-confirm state deterministically instead
-  /// of racing real stream/timer scheduling. Complete it to let the call
-  /// proceed.
-  Completer<void>? puertaConfirmarAudioFlutter;
-
-  /// Test-only failure switch (Slice 3 edge case): when true,
-  /// [confirmarAudioFlutter] throws after the gate above (if any) resolves,
-  /// simulating a dead/never-there native channel at handoff.
-  bool fallaConfirmarAudioFlutter = false;
-
   /// Simulates a native -> Flutter `alarmFired` MethodChannel event.
   void emitirEvento(EventoAlarmaAndroid evento) => _eventos.add(evento);
 
@@ -74,27 +54,6 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
     ocultadas.add(alarmaId);
   }
 
-  @override
-  Future<void> confirmarAudioFlutter(String alarmaId) async {
-    if (puertaConfirmarAudioFlutter != null) {
-      await puertaConfirmarAudioFlutter!.future;
-    }
-    if (fallaConfirmarAudioFlutter) {
-      throw StateError('fake confirmarAudioFlutter failure');
-    }
-    detenidas.add(alarmaId);
-  }
-
-  @override
-  Future<void> forzarVolumenMediaParaAlarma(double fraccion) async {
-    volumenForzado.add(fraccion);
-  }
-
-  @override
-  Future<void> restaurarVolumenMedia() async {
-    volumenRestaurado++;
-  }
-
   @override
   Future<DiagnosticoAlarmasAndroid> diagnostico() async =>
       DiagnosticoAlarmasAndroid(