feat(exécution): chrono du score chronométré pendant la séance (ticket #27)

Ajoute le chrono du score chronométré (fondation domain/config des
tickets #25/#26) sur workout_execution_screen.dart, avec les use
cases associés. flutter analyze propre, 75/75 tests verts, build APK
debug validé.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

lib/application/use_cases.dart

@@ -1188,6 +1188,24 @@ final class ActiveWorkoutSessionUseCases {
     );
   }
 
+  Future<ActiveScoreStopwatchState?> findScoreStopwatch({
+    required String sessionId,
+    required int programIndex,
+    required int exerciseIndex,
+    required int setIndex,
+  }) {
+    return sessionRepository.findScoreStopwatchState(
+      sessionId: sessionId,
+      programIndex: programIndex,
+      exerciseIndex: exerciseIndex,
+      setIndex: setIndex,
+    );
+  }
+
+  int scoreStopwatchElapsedMilliseconds(ActiveScoreStopwatchState state) {
+    return state.elapsedMillisecondsAt(clock.now());
+  }
+
   Future<List<SetResultPositionState>> listSetResults(String sessionId) async {
     final session = await _requiredSession(sessionId);
     final snapshots = _listSetSnapshots(session.resolvedTemplateSnapshotJson);