Stage-1 RVQ for Twi (Whisper encoder features)

Single RVQ wrapper (no LayerNorms) with ema_counts buffer for parity
Files: config_stage1.json, rvq_final.pt, rvq_best.pt (if any), rvq_averaged_full.pt, rvq_wrapper.py
Decoding uses Whisper generate with timestamps disabled and no forced decoder ids (transcribe mode).