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>

web/engine.sx

@@ -125,10 +125,14 @@
         (when target-sel
           (dict-set! headers "SX-Target" target-sel)))
 
-      ;; Loaded component names
-      (when (not (empty? loaded-components))
-        (dict-set! headers "SX-Components"
-          (join "," loaded-components)))
+      ;; Send component hash instead of full name list to avoid 431
+      ;; (Request Header Fields Too Large) with many loaded components.
+      ;; Server uses hash to decide whether to send component definitions.
+      (let ((comp-hash (dom-get-attr
+                         (dom-query "script[data-components][data-hash]")
+                         "data-hash")))
+        (when comp-hash
+          (dict-set! headers "SX-Components-Hash" comp-hash)))
 
       ;; CSS class hash
       (when css-hash