Add resource SSR primitive, restore live demos in Examples section

- sx_primitives.ml: register resource as SSR stub returning signal
  {loading: true} — server renders loading state, client hydrates fetch
- reactive-islands/index.sx: Examples section with all 15 demo islands
  rendered inline alongside highlighted source code

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

hosts/ocaml/lib/sx_primitives.ml

@@ -686,6 +686,18 @@ let () =
        | None -> raise (Eval_error ("Store not found: " ^ name)))
     | _ -> raise (Eval_error "use-store: expected (name)"));
   register "clear-stores" (fun _args -> Hashtbl.clear store_registry; Nil);
+  (* resource — SSR stub: return signal with {loading: true}, client hydrates real fetch *)
+  register "resource" (fun _args ->
+    let state = Hashtbl.create 8 in
+    Hashtbl.replace state "loading" (Bool true);
+    Hashtbl.replace state "data" Nil;
+    Hashtbl.replace state "error" Nil;
+    let sig_d = Hashtbl.create 8 in
+    Hashtbl.replace sig_d "__signal" (Bool true);
+    Hashtbl.replace sig_d "value" (Dict state);
+    Hashtbl.replace sig_d "subscribers" (List []);
+    Hashtbl.replace sig_d "deps" (List []);
+    Dict sig_d);
   register "apply" (fun args ->
     let call f a =
       match f with