fix: implement debounced barge-in check, greeting isolation, and remove aggressive dynamic VAD fast-tracking to eliminate premature filler interruptions and cut-off greetings

feat: parallel TTS, pre-synthesized greeting, edge cases, dynamic VAD

fix: VAD singleton + timer-based listening reset + wrapped init in try/except

fix: implement barge-in grace period and increase VAD threshold to prevent echo-interrupts

feat(E1): neural Silero VAD + Sarvam TTS + language-aware silence