feat: Add AD-23 Phase-1 distillation, expert cache, and DDD updates

AD-23: Phase-1 Distillation via External GPU Teacher Artifacts
- One-time GPU job produces behavioral artifacts (routing traces,
  sparse logits, preference labels) — not trained weights
- CPU-only refinement: router repair, LoRA correction, EWC++, policy
  optimization using teacher artifacts
- Acceptance criteria: 200-prompt suite, all 3 behavioral gates,
  stability under 10% corpus perturbation

expert_cache.rs: MoE expert hot-set caching (new file)
- ExpertCache with LRU/LFU/Adaptive eviction policies
- MoeBatchScheduler: reorder token execution by expert for cache reuse
- Prefetcher trait for future platform-specific prefetch intrinsics
- 12 tests (92/92 bitnet tests pass)

DDD v2.5: 6 new ubiquitous language terms (Teacher Artifact, Behavioral
Distillation, Router Repair, Sparse Logits, Corpus Perturbation) and
4 new open questions (#27-30) for Phase-1 operability.

https://claude.ai/code/session_011nTcGcn49b8YKJRVoh4TaK

Author: claude