fix(alarma-sonando): restyle Stop as a neutral translucent surface

Audit 9.11 (t4 line 434): the Stop button was a FilledButton in
colorScheme.primary (brand cyan), the wrong colour family for the
largest element on the ringing screen. The prototype draws a neutral
rgba(255,255,255,.08) surface with a rgba(255,255,255,.16) border and
radius 24 (none of PluriWaveTokens' three named radii).

test/pantallas/pantalla_alarma_sonando_test.dart

@@ -235,6 +235,53 @@ void main() {
     });
   });
 
+  group('visual fidelity (audit 9.11, proto t4 line 434): Detener es una '
+      'superficie neutra translucida, no cian', () {
+    testWidgets(
+      'fondo blanco 8%, borde blanco 16%, radio 24 — no colorScheme.primary/'
+      'radiusLg',
+      (tester) async {
+        await _montarPantalla(tester);
+
+        final elemento = find.byKey(const ValueKey('ringing-stop-button'));
+        final boton = tester.widget<FilledButton>(elemento);
+        final contexto = tester.element(elemento);
+        final primary = Theme.of(contexto).colorScheme.primary;
+
+        final fondo = boton.style?.backgroundColor?.resolve(<WidgetState>{});
+        final borde = boton.style?.side?.resolve(<WidgetState>{});
+        final forma =
+            boton.style?.shape?.resolve(<WidgetState>{})
+                as RoundedRectangleBorder?;
+
+        expect(
+          fondo,
+          Colors.white.withValues(alpha: 0.08),
+          reason:
+              'prototype t4 line 434: rgba(255,255,255,.08) — a neutral '
+              'translucent surface',
+        );
+        expect(
+          fondo,
+          isNot(primary),
+          reason:
+              'the largest element on a full-screen surface must not stay '
+              'the brand cyan colorScheme.primary (PluriWaveTokens.brand)',
+        );
+        expect(
+          borde?.color,
+          Colors.white.withValues(alpha: 0.16),
+          reason: 'prototype t4 line 434 border: rgba(255,255,255,.16)',
+        );
+        expect(
+          forma?.borderRadius,
+          BorderRadius.circular(24),
+          reason: 'prototype t4 line 434: radius 24 (was radiusLg — 30)',
+        );
+      },
+    );
+  });
+
   group('WU11 — estado estatico de subida de volumen (resolucion 4, sin '
       'contador en vivo)', () {
     testWidgets(