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/templates/shell.sx

@@ -15,6 +15,7 @@
                                (sx-css :as string?) (sx-css-classes :as string?)
                                (component-hash :as string?) (component-defs :as string?)
                                (pages-sx :as string?) (page-sx :as string?)
+                               (body-html :as string?)
                                (asset-url :as string) (sx-js-hash :as string) (body-js-hash :as string?)
                                (head-scripts :as list?) (inline-css :as string?) (inline-head-js :as string?)
                                (init-sx :as string?) (body-scripts :as list?))
@@ -65,6 +66,8 @@ details.group{overflow:hidden}details.group>summary{list-style:none}details.grou
 .sx-error .sx-indicator{display:none}.sx-loading .sx-indicator{display:inline-flex}
 .js-wrap.open .js-pop{display:block}.js-wrap.open .js-backdrop{display:block}"))))
       (body :class "bg-stone-50 text-stone-900"
+        ;; Server-rendered HTML — visible immediately before JS loads
+        (div :id "sx-root" (raw! (or body-html "")))
         (script :type "text/sx" :data-components true :data-hash component-hash
           (raw! (or component-defs "")))
         (when init-sx
@@ -72,7 +75,7 @@ details.group{overflow:hidden}details.group>summary{list-style:none}details.grou
             (raw! init-sx)))
         (script :type "text/sx-pages"
           (raw! (or pages-sx "")))
-        (script :type "text/sx" :data-mount "body"
+        (script :type "text/sx" :data-mount "#sx-root"
           (raw! (or page-sx "")))
         (script :src (str asset-url "/scripts/sx-browser.js?v=" sx-js-hash))
         ;; Body scripts — configurable per app