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/sx/ocaml_bridge.py

@@ -397,10 +397,11 @@ class OcamlBridge:
             # All directories loaded into the Python env
             all_dirs = list(set(_watched_dirs) | _dirs_from_cache)
 
-            # Web adapters (aser lives in adapter-sx.sx) — only load specific files
+            # Web adapters + signals (aser lives in adapter-sx.sx,
+            # signals.sx provides reactive primitives for island SSR)
             web_dir = os.path.join(os.path.dirname(__file__), "../../web")
             if os.path.isdir(web_dir):
-                for web_file in ["adapter-sx.sx"]:
+                for web_file in ["signals.sx", "adapter-sx.sx"]:
                     path = os.path.normpath(os.path.join(web_dir, web_file))
                     if os.path.isfile(path):
                         all_files.append(path)