Defer content initialization during dev startup - #17975

Changes

• Moves development content store loading, type generation, and content synchronization off the server-ready critical path.
• Gates Astro SSR and prerender requests on content readiness while allowing Vite static/internal requests through, and exposes contentReady on the experimental JavaScript dev server for consumers that need the initial generated artifacts.
• Settles background content work before restart or shutdown.

Testing

• Adds a dev content deferral fixture covering fresh-store first requests, ungated static assets, immediate shutdown, config restart, and broken content configuration.
• Updates the Cloudflare chunked-content test to explicitly await content readiness before inspecting its generated manifest.

Docs

• No docs update needed because the only API addition is on the experimental JavaScript dev server.