Design lazy renderer preload for streaming SSR

[woksin]

Problem

Renderer slot entries are synchronous in Components 4. preloadRenderer and lazy slot entries were deliberately deferred because streaming SSR and hydration determinism were not proven. Inventing a load contract without a real stream fixture would create an unstable cache/protocol surface.

Direction

Design lazy renderer loading around a real streaming SSR consumer:

• preload resolves lazy entries into a synchronous request-local cache before renderToPipeableStream;
• server serializes renderer/profile identity and the client verifies the same identity before hydration;
• concurrent requests cannot share mutable renderer state;
• rejected/aborted loads, missing chunks, CSP, timeout, retry, and fallback semantics are explicit;
• zero-config and eager adapters pay no runtime/bundle penalty.

Acceptance criteria

• A documented streaming SSR fixture preloads, streams, and hydrates with zero structural diff.
• Concurrent requests selecting different renderers remain isolated.
• Missing/rejected loads fail with one actionable Core-owned diagnostic.
• Client/server renderer-id mismatch fails before corrupt hydration.
• The eager path and setup-only root remain unchanged.
• Only then are lazy slot entries and preload APIs eligible for promotion.