feat(turbo-kv): stage 2 — activate compressed KV attention pipeline

Full TurboKV pipeline now operational:
  - Encode history → 3-bit PolarQuant (existing)
  - Truncate hot cache to hot window (new)
  - Decode + prepend history at every SDPA call (new)

RAM savings: ~5.5x on history tokens (3-bit vs 16-bit fp16)

Author: solderzzc