Isomorphic SSR: server renders HTML body, client takes over with SX

Server now renders page content as HTML inside <div id="sx-root">,
visible immediately before JavaScript loads. The SX source is still
included in a <script data-mount="#sx-root"> tag for client hydration.

SSR pipeline: after aser produces the SX wire format, parse and
render-to-html it (~17ms for a 22KB page). Islands with reactive
state gracefully fall back to empty — client hydrates them.

Supporting changes:
- Load signals.sx into OCaml kernel (reactive primitives for island SSR)
- Add cek-call and context to kernel env (needed by signals/deref)
- Island-aware component accessors in sx_types.ml
- render-to-html handles Island values (renders as component with fallback)
- Fix 431 (Request Header Fields Too Large): replace SX-Components
  header (full component name list) with SX-Components-Hash (12 chars)
- CORS allow SX-Components-Hash header

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/infrastructure/factory.py

@@ -344,7 +344,7 @@ def create_base_app(
             response.headers["Access-Control-Allow-Origin"] = origin
             response.headers["Access-Control-Allow-Credentials"] = "true"
             response.headers["Access-Control-Allow-Headers"] = (
-                "SX-Request, SX-Target, SX-Current-URL, SX-Components, SX-Css, "
+                "SX-Request, SX-Target, SX-Current-URL, SX-Components, SX-Components-Hash, SX-Css, "
                 "HX-Request, HX-Target, HX-Current-URL, HX-Trigger, "
                 "Content-Type, X-CSRFToken"
             )