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/lib/sx_render.ml

@@ -256,17 +256,20 @@ 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. *)
+       | Island _i ->
+         (* Islands: SSR via the SX render-to-html from adapter-html.sx.
+            It handles deref/signal/computed through the CEK correctly,
+            and renders island bodies with hydration markers. *)
          (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
+           let call_expr = List (Symbol name :: args) in
+           let quoted = List [Symbol "quote"; call_expr] in
+           let render_call = List [Symbol "render-to-html"; quoted; Env env] in
+           let result = Sx_ref.eval_expr render_call (Env env) in
+           (match result with
+            | String s | RawHTML s -> s
+            | _ -> value_to_string result)
          with e ->
-           Printf.eprintf "[ssr-island] ~%s FAILED: %s\n%!" i.i_name (Printexc.to_string e);
+           Printf.eprintf "[ssr-island] ~%s FAILED: %s\n%s\n%!" _i.i_name (Printexc.to_string e) (Printexc.get_backtrace ());
            "")
        | Macro m ->
          let expanded = expand_macro m args env in