docs: record workspace membership admin merge

[yha9806]

Summary

• Record platform PR Add Seattle Polish Festival visual spec #41 as the latest merged Workspace membership admin slice.
• Update the vault manifest, source index, durable Workspace status, release-readiness gate/spec, and M5 closeout.
• Preserve the boundary that Add Seattle Polish Festival visual spec #41 is system-only compatibility-route membership provisioning, not full user/JWT auth, self-service membership UI, typed aggregates, ingress proof, multi-instance acceptance, or R5 release readiness.

Verification

• python3 docs/review-context/gates/validate_review_context.py
• python3 -m json.tool docs/review-context/MANIFEST.json
• python3 -m json.tool docs/review-context/release-readiness/m5-closeout-summary.json
• git diff --check --cached
• git diff --cached | gemini-agent diff-review --stdin