Document movable collection authority

[callumalpass]

What changed

• add a public “Why one movable authority?” section to the Connect overview
• explain the continuous coordination costs an equal-peer design would impose on ordinary Markdown writers
• document authority transfer as an identity-preserving, epoch-fenced role change
• migrate SDK documentation from the removed hostedSync() and hosted route names to provider-neutral sync() and remote
• synchronize the current Connect protocol schema and pin CI/deployment to the merged authority-transfer sources

Why

mdbase deliberately uses one movable logical authority with writable mirrors. The site should explain that this preserves open filesystem editing while avoiding permanent CRDT, split-brain, path-conflict, and peer-membership machinery.

Validation

• canonical source synchronization
• Astro check: 0 errors and 0 warnings
• static production build: 22 pages
• local-link validation: 24 HTML files with no broken links
• legacy API-name audit