Island SSR: defislands render to HTML server-side with hydration markers

Islands now render their initial state as HTML on the server, like
React SSR. The client hydrates with reactive behavior on boot.

Root causes fixed:
- is_signal/signal_value now recognize Dict-based signals (from
  signals.sx) in addition to native Signal values
- Register "context" as a primitive so the CEK deref frame handler
  can read scope stacks for reactive tracking
- Load adapter-html.sx into kernel for SX-level render-to-html
  (islands use this instead of the OCaml render module)
- Component accessors (params, body, has-children?, affinity) handle
  Island values with ? suffix aliases
- Add platform primitives: make-raw-html, raw-html-content,
  empty-dict?, for-each-indexed, cek-call

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

hosts/ocaml/bin/sx_server.ml

@@ -150,6 +150,16 @@ let () = Sx_primitives.register "scope-peek" (fun args ->
     (match stack with v :: _ -> v | [] -> Nil)
   | _ -> Nil)
 
+let () = Sx_primitives.register "context" (fun args ->
+  match args with
+  | [String name] | [String name; _] ->
+    let stack = try Hashtbl.find _scope_stacks name with Not_found -> [] in
+    (match stack, args with
+     | v :: _, _ -> v
+     | [], [_; default_val] -> default_val
+     | [], _ -> Nil)
+  | _ -> Nil)
+
 (** collect! — lazy scope accumulator. Creates root scope if missing,
     emits value (deduplicates).  Used by cssx and spread components. *)
 let () = Sx_primitives.register "collect!" (fun args ->
@@ -376,13 +386,29 @@ let make_server_env () =
     | [List items; v] -> List (items @ [v])
     | _ -> raise (Eval_error "append!: expected list and value"));
 
-  (* HTML renderer *)
+  (* HTML renderer — OCaml render module provides the shell renderer;
+     adapter-html.sx provides the SX-level render-to-html *)
   Sx_render.setup_render_env env;
 
   (* Render-mode flags *)
   bind "set-render-active!" (fun _args -> Nil);
   bind "render-active?" (fun _args -> Bool true);
 
+  (* Raw HTML — platform primitives for adapter-html.sx *)
+  bind "make-raw-html" (fun args ->
+    match args with [String s] -> RawHTML s | [v] -> RawHTML (value_to_string v) | _ -> Nil);
+  bind "raw-html-content" (fun args ->
+    match args with [RawHTML s] -> String s | [String s] -> String s | _ -> String "");
+  bind "empty-dict?" (fun args ->
+    match args with [Dict d] -> Bool (Hashtbl.length d = 0) | _ -> Bool true);
+  bind "for-each-indexed" (fun args ->
+    match args with
+    | [fn_val; List items] | [fn_val; ListRef { contents = items }] ->
+      List.iteri (fun i item ->
+        ignore (Sx_ref.eval_expr (List [fn_val; Number (float_of_int i); item]) (Env env))
+      ) items; Nil
+    | _ -> Nil);
+
   (* Scope stack — platform primitives for render-time dynamic scope.
      Used by aser for spread/provide/emit patterns.
      Module-level so step-sf-context can check it via get-primitive. *)
@@ -764,21 +790,27 @@ let make_server_env () =
   bind "component-params" (fun args ->
     match args with
     | [Component c] -> List (List.map (fun s -> String s) c.c_params)
+    | [Island i] -> List (List.map (fun s -> String s) i.i_params)
     | _ -> Nil);
 
   bind "component-body" (fun args ->
     match args with
     | [Component c] -> c.c_body
+    | [Island i] -> i.i_body
     | _ -> Nil);
 
-  bind "component-has-children" (fun args ->
+  let has_children_impl = NativeFn ("component-has-children?", fun args ->
     match args with
     | [Component c] -> Bool c.c_has_children
-    | _ -> Bool false);
+    | [Island i] -> Bool i.i_has_children
+    | _ -> Bool false) in
+  ignore (env_bind env "component-has-children" has_children_impl);
+  ignore (env_bind env "component-has-children?" has_children_impl);
 
   bind "component-affinity" (fun args ->
     match args with
     | [Component c] -> String c.c_affinity
+    | [Island _] -> String "client"
     | _ -> String "auto");
 
   bind "keyword-name" (fun args ->