feat(streaming): buffer resilience and automatic reconnection

- Construct the audio player with an enlarged live-stream buffer (15-50s forward cushion, 2.5s to start, 5s after rebuffer) so short network drops play through silently
- Add reconnect-on-stall state machine with bounded exponential backoff (1/2/4/8/16s, ~90s total window, 5 attempts) that re-prepares to the live edge; backoff/decision logic extracted to controlador_reconexion.dart as pure testable code
- Surface a new reconnecting playback state in the mini player and full player (localized in all 13 locales) instead of error dialogs during the retry window; a single friendly error appears only after exhaustion
- Guard interplay: user pause/stop cancels retries, audio interruptions cancel reconnect, alarm wake-up path keeps precedence, recording fails cleanly during drops
- Reset retry budget on station change; route stream timeouts through the network-error class
- 10 new tests (99 total green), flutter analyze clean

lib/servicios/servicio_grabacion_radio.dart

@@ -284,6 +284,10 @@ class ServicioGrabacionRadio {
     _emitir(const EstadoGrabacionRadio.inactiva());
   }
 
+  // S7-R5 invariant: recording uses its OWN HTTP stream, independent of
+  // PluriWaveAudioHandler. Its error handling must NOT route through the S7
+  // reconnect machine (ControladorReconexion) — a recording failure clears
+  // state and releases resources immediately, with no backoff retries.
   Future<void> _fallar(Object error) async {
     _timerAutoStop?.cancel();
     _timerAutoStop = null;