Island SSR: spreads work, CSS classes render, context primitive registered

Root causes:
- make-spread/spread?/spread-attrs were stubbed (always false/empty)
  → now create/detect/unwrap Spread values properly
- "context" primitive missing from Sx_primitives registry
  → CEK deref frame handler couldn't read reactive scope stacks

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

hosts/ocaml/bin/sx_server.ml

@@ -565,8 +565,21 @@ let make_server_env () =
     | [Component c] -> Env c.c_closure
     | [Island i] -> Env i.i_closure
     | _ -> Dict (Hashtbl.create 0));
-  bind "spread?" (fun _args -> Bool false);
-  bind "spread-attrs" (fun _args -> Dict (Hashtbl.create 0));
+  bind "spread?" (fun args ->
+    match args with [Spread _] -> Bool true | _ -> Bool false);
+  bind "spread-attrs" (fun args ->
+    match args with
+    | [Spread pairs] ->
+      let d = Hashtbl.create 4 in
+      List.iter (fun (k, v) -> Hashtbl.replace d k v) pairs;
+      Dict d
+    | _ -> Dict (Hashtbl.create 0));
+  bind "make-spread" (fun args ->
+    match args with
+    | [Dict d] ->
+      let pairs = Hashtbl.fold (fun k v acc -> (k, v) :: acc) d [] in
+      Spread pairs
+    | _ -> Nil);
   bind "is-html-tag?" (fun args ->
     match args with
     | [String s] -> Bool (Sx_render.is_html_tag s)