[codex] Clarify prefab component reference docs

[mfoltz]

Summary

• Clarify the prefab Components relation surface with a short extracted-snapshot note.
• Distinguish linked component docs from attached components without generated docs.
• Add a Component Docs summary stat and focused ReferenceDetailView coverage.

Validation

• npm run test
• npm run verify

Notes

• This branch was created cleanly from origin/main to avoid coupling the clarity pass to the roadmap-spacing branch history.