docs(rfc): mark m6 mostly complete with r4 deferred

[huangruiteng]

Summary

Mark M6 as mostly complete with R4 explicitly deferred.

• Update milestone status to reflect R1-R3 runtime replacements and the
  deferred R4 executor.
• Add a Replacement Status subsection listing the merged replacement PRs.

No runtime behavior changes.

Validation

• docs-governance-smoke: passed.
• loopx canary premerge --from-git-diff: passed, self_merge_allowed=true,
  0 manual holds.
• Public boundary scan: passed.

Routing

• continuation-policy: independent_handoff
• excluded-agent: codex-quality-qualification
• claimed-by: codex-side-bypass

[huangruiteng]

Approval conclusion (author-owned PR; GitHub blocks formal self-approval)

Reviewed exact head 4643f86. No blocking findings.

The RFC marks M6 mostly complete after R1-R3 and keeps R4 deferred. Docs only,
no runtime behavior changes.

Validation

• docs-governance-smoke: passed.
• loopx canary premerge --from-git-diff: passed, self_merge_allowed=true,
  0 manual holds.
• GitHub dependency-review on this head: pass. Python/build workflows are not
  triggered for a docs-only path change.
• Public boundary scan: passed.

Merge decision: approved after reviewer.