docs(rfc): define NoKV storage and LoopX authority boundaries

[huangruiteng]

Summary

• define the durable three-layer boundary: NoKV storage plane, LoopX semantic authority, and command-driven reconciliation
• separate coordination-head credentials from checkpoint and evidence artifact publication
• add a capability horizon from deterministic claims through recoverable ownership, continuous supervision, and service-grade deployment
• clarify that co-deployment is allowed and that a thin transport wrapper alone does not justify a repository, product, or license split
• keep the English and Chinese RFCs as semantic mirrors

Validation

• Python 3.12: python examples/nokv-shadow-provider/probes.py contract
• LoopX public-boundary scan: clean for both changed RFC files; unrelated global local-state warnings are outside this diff
• uv run --no-project --with-requirements docs/requirements-docs.txt mkdocs build --strict
• git diff --check

The host Python 3.9 is below the syntax and stdlib level used by current LoopX, so project checks were rerun with Python 3.12 and passed. No runtime behavior changes.

Refs #3245.