Document evidence-gated capability transfer

[clocksmith]

Summary

• rename the broad hybrid-distillation method to Evidence-Gated Capability Transfer (EGCT)
• define experiment, evidence, promotion, serving, and receipt contracts in a canonical guide
• retain HYBRID_DISTILLATION.md as a compatibility pointer
• document how EGCT relates to SRSTC and incorporate the latest WGSL GRPO/DPO evidence

Verification

• .venv/bin/python -m pytest tests/test_docs_cli_parity.py -m docs -q (6 passed)
• git diff --check origin/main...HEAD