[codex] Document registry identifier contracts

[jeremi]

Summary

• Add the deferred identifier domain consolidation plan under lab/docs and link it from the Lab docs index.
• Refresh the contracts and error reference review metadata while preserving the current main wording for provenance-enabled Relay routes and Notary operational problem codes.

Why

The existing local changes documented useful machine-identifier and problem-code follow-up work, but the plan was not discoverable from the Lab docs index and the PR needed a clean branch that does not carry older merged work.

Impact

Docs-only. This does not change runtime behavior, generated artifacts, or product code.

Validation

• git diff --check origin/main...HEAD
• npm run check:content
• npm run check:markdown
• docs/site/node_modules/.bin/markdownlint-cli2 lab/docs/README.md lab/docs/domain-consolidation-plan.md
• npm run build