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>

hosts/ocaml/bin/sx_server.ml

@@ -425,7 +425,7 @@ let make_server_env () =
   (* context — scope lookup.  The CEK handles this as a special form
      by walking continuation frames, but compiled VM code needs it as
      a function that reads from the scope_stacks hashtable. *)
-  bind "sx-context" (fun args ->
+  let context_impl = NativeFn ("context", fun args ->
     match args with
     | [String name] | [String name; _] ->
       let stack = try Hashtbl.find scope_stacks name with Not_found -> [] in
@@ -433,7 +433,9 @@ let make_server_env () =
        | v :: _, _ -> v
        | [], [_; default_val] -> default_val
        | [], _ -> Nil)
-    | _ -> Nil);
+    | _ -> Nil) in
+  ignore (env_bind env "sx-context" context_impl);
+  ignore (env_bind env "context" context_impl);
 
   (* qq-expand-runtime — quasiquote expansion at runtime.
      The bytecode compiler emits CALL_PRIM "qq-expand-runtime" for
@@ -464,6 +466,12 @@ let make_server_env () =
     | [fn_val; List call_args] ->
       Sx_ref.eval_expr (List (fn_val :: call_args)) (Env env)
     | _ -> raise (Eval_error "call-lambda: expected (fn args env?)"));
+  bind "cek-call" (fun args ->
+    match args with
+    | [fn_val; List call_args] -> Sx_ref.cek_call fn_val (List call_args)
+    | [fn_val; Nil] -> Sx_ref.cek_call fn_val (List [])
+    | [fn_val] -> Sx_ref.cek_call fn_val (List [])
+    | _ -> Nil);
   bind "expand-macro" (fun args ->
     match args with
     | [Macro m; List macro_args; Env e] ->
@@ -1072,6 +1080,22 @@ let rec dispatch env cmd =
       in
       let body_final = flush_batched_io body_str in
       let t2 = Unix.gettimeofday () in
+      (* Phase 1b: render the aser'd SX to HTML for isomorphic SSR.
+         The aser output is flat (all components expanded, just HTML tags),
+         so render-to-html is cheap — no component lookups needed. *)
+      let body_html =
+        try
+          let body_exprs = Sx_parser.parse_all body_final in
+          let body_expr = match body_exprs with
+            | [e] -> e | [] -> Nil | _ -> List (Symbol "<>" :: body_exprs)
+          in
+          Sx_render.render_to_html body_expr env
+        with e ->
+          Printf.eprintf "[ssr] render-to-html failed: %s\n%!" (Printexc.to_string e);
+          ""  (* fallback: client renders from SX source. Islands with
+                 reactive state may fail SSR — client hydrates them. *)
+      in
+      let t2b = Unix.gettimeofday () in
       (* Phase 2: render shell with body + all kwargs.
          Resolve symbol references (e.g. __shell-component-defs) to their
          values from the env — these were pre-injected by the bridge. *)
@@ -1082,13 +1106,15 @@ let rec dispatch env cmd =
            with _ -> try Sx_primitives.get_primitive s with _ -> v)
         | _ -> v
       ) shell_kwargs in
-      let shell_args = Keyword "page-sx" :: String body_final :: resolved_kwargs in
+      let shell_args = Keyword "page-sx" :: String body_final
+                     :: Keyword "body-html" :: String body_html
+                     :: resolved_kwargs in
       let shell_call = List (Symbol "~shared:shell/sx-page-shell" :: shell_args) in
       let html = Sx_render.render_to_html shell_call env in
       let t3 = Unix.gettimeofday () in
-      Printf.eprintf "[sx-page-full] aser=%.3fs io=%.3fs shell=%.3fs total=%.3fs body=%d html=%d\n%!"
-        (t1 -. t0) (t2 -. t1) (t3 -. t2) (t3 -. t0)
-        (String.length body_final) (String.length html);
+      Printf.eprintf "[sx-page-full] aser=%.3fs io=%.3fs ssr=%.3fs shell=%.3fs total=%.3fs body=%d ssr=%d html=%d\n%!"
+        (t1 -. t0) (t2 -. t1) (t2b -. t2) (t3 -. t2b) (t3 -. t0)
+        (String.length body_final) (String.length body_html) (String.length html);
       send_ok_string html
     with
     | Eval_error msg ->

hosts/ocaml/lib/sx_render.ml

@@ -256,6 +256,18 @@ and render_list_to_html head args env =
       let v = env_get env name in
       (match v with
        | Component _ -> render_component v args env
+       | Island i ->
+         (* Islands: render initial HTML server-side (like React SSR).
+            Log failures so we can fix them. *)
+         (try
+           let c = { c_name = i.i_name; c_params = i.i_params;
+                     c_has_children = i.i_has_children; c_body = i.i_body;
+                     c_closure = i.i_closure; c_affinity = "client";
+                     c_compiled = None } in
+           render_component (Component c) args env
+         with e ->
+           Printf.eprintf "[ssr-island] ~%s FAILED: %s\n%!" i.i_name (Printexc.to_string e);
+           "")
        | Macro m ->
          let expanded = expand_macro m args env in
          do_render_to_html expanded env

hosts/ocaml/lib/sx_types.ml

@@ -347,26 +347,32 @@ let set_lambda_name l n = match l with
 
 let component_name = function
   | Component c -> String c.c_name
+  | Island i -> String i.i_name
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_params = function
   | Component c -> List (List.map (fun s -> String s) c.c_params)
+  | Island i -> List (List.map (fun s -> String s) i.i_params)
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_body = function
   | Component c -> c.c_body
+  | Island i -> i.i_body
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_closure = function
   | Component c -> Env c.c_closure
+  | Island i -> Env i.i_closure
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_has_children = function
   | Component c -> Bool c.c_has_children
+  | Island i -> Bool i.i_has_children
   | v -> raise (Eval_error ("Expected component, got " ^ type_of v))
 
 let component_affinity = function
   | Component c -> String c.c_affinity
+  | Island _ -> String "client"
   | _ -> String "auto"
 
 let macro_params = function