[codex] Document mergeable containers

[zxch3n]

Summary

• Document the new LoroMap.ensureMergeable* APIs across API, concept, tutorial, CID, and LLM-facing docs.
• Mark getOrCreateContainer as deprecated for lazily-created Map child containers and point readers to ensureMergeable*.
• Clarify that mergeable container IDs use a special Root Container ID whose stable name is derived from (parent Map, key, container type).
• Regenerate public/llms-full.txt and update the blog RSS title/content generated from the mergeable containers post.

Validation

• pnpm extract-llms
• pnpm build exited 0. Existing warnings remain for Browserslist data and loro_wasm async/wasm target support; Wrangler also logged a sandbox-only EPERM while trying to write to ~/Library/Preferences/.wrangler/logs, but OpenNext completed successfully.