/// Interface abstraite pour la synthèse vocale (Text-to-Speech).
/// Implémentations disponibles :
///   - FlutterTtsEngine (Google on-device — gratuit, FR/NL/EN/DE)
///   - ElevenLabsTtsEngine (ElevenLabs API — voix naturelle, payant)
///   - AzureTtsEngine (upgrade futur)
abstract class TtsEngine {
  Future<void> speak(String text, {String languageCode = 'fr-FR'});
  Future<void> stop();
  Future<void> replay();

  bool get isSpeaking;
}