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/helpers.py

@@ -521,8 +521,18 @@ def components_for_request(source: str = "",
     elif extra_names:
         needed = extra_names
 
-    loaded_raw = request.headers.get("SX-Components", "")
-    loaded = set(loaded_raw.split(",")) if loaded_raw else set()
+    # Check hash first (new): if client hash matches current, skip all defs.
+    # Fall back to legacy name list (SX-Components) for backward compat.
+    comp_hash_header = request.headers.get("SX-Components-Hash", "")
+    if comp_hash_header:
+        from .jinja_bridge import components_for_page
+        _, current_hash = components_for_page("", service=None)
+        if comp_hash_header == current_hash:
+            return ""  # client has everything
+        loaded = set()  # hash mismatch — send all needed
+    else:
+        loaded_raw = request.headers.get("SX-Components", "")
+        loaded = set(loaded_raw.split(",")) if loaded_raw else set()
 
     parts = []
     for key, val in _COMPONENT_ENV.items():

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)

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