fix(alarm): defer the Dart fade-in until the native handoff confirms

The native service and the Flutter player each ran their own 5%-to-
target fade-in, and both could drive audible volume at the handoff,
producing a jump or ramp reset. The Dart ramp now starts exactly once
from the handoff-confirmation path: the player still pre-starts at 5%,
and _confirmarAudioFlutterListo() starts the ramp in a finally block
so it runs whether the native confirmation succeeds or fails — the
alarm can never stay stuck at 5% if the native side is already gone.

Work unit 3/3 of alarm-volume-ramp-restore (fade-in dedup).

test/helpers/fakes_alarmas.dart

@@ -30,6 +30,18 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
   /// 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);
 
@@ -64,6 +76,12 @@ class FakePuertoAlarmasAndroid implements PuertoAlarmasAndroid {
 
   @override
   Future<void> confirmarAudioFlutter(String alarmaId) async {
+    if (puertaConfirmarAudioFlutter != null) {
+      await puertaConfirmarAudioFlutter!.future;
+    }
+    if (fallaConfirmarAudioFlutter) {
+      throw StateError('fake confirmarAudioFlutter failure');
+    }
     detenidas.add(alarmaId);
   }