Live demo islands with highlighted source: SSR-safe effect, native component-source

- signals.sx: guard effect body with (when (client?) ...) so effects
  are no-op during SSR — only 2 stubs needed (effect, register-in-scope)
- sx_primitives.ml: add resource SSR stub (returns signal {loading: true}),
  remove 27 unnecessary browser primitive stubs
- sx_server.ml: native component-source that looks up Component/Island
  from env and pretty-prints the definition (replaces broken Python helper)
- reactive-islands/index.sx: Examples section with all 15 live demos
  inline + highlighted source via component-source
- reactive-islands/demo.sx: replace 14 hardcoded highlight strings with
  (component-source "~name") calls for always-current source

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

hosts/ocaml/bin/sx_server.ml

@@ -2015,6 +2015,30 @@ let http_setup_page_helpers env =
         Nil)
     | _ -> raise (Eval_error "helper: expected (helper \"name\" ...args)"));
 
+  (* component-source — look up component/island from env, pretty-print its definition *)
+  bind "component-source" (fun args ->
+    match args with
+    | [String name] ->
+      let lookup = if String.length name > 0 && name.[0] = '~'
+                   then name
+                   else "~" ^ name in
+      (try
+        let comp = env_get env lookup in
+        match comp with
+        | Component c ->
+          let params = List (List.map (fun s -> Symbol s) c.c_params) in
+          let form = List [Symbol "defcomp"; Symbol ("~" ^ c.c_name);
+                           params; c.c_body] in
+          String (pretty_print_value form)
+        | Island c ->
+          let params = List (List.map (fun s -> Symbol s) c.i_params) in
+          let form = List [Symbol "defisland"; Symbol ("~" ^ c.i_name);
+                           params; c.i_body] in
+          String (pretty_print_value form)
+        | _ -> String (";; " ^ name ^ ": not a component")
+      with _ -> String (";; component " ^ name ^ " not found"))
+    | _ -> raise (Eval_error "component-source: expected (name)"));
+
   (* Stub remaining demo/action helpers that need real IO *)
   let stub name = bind name (fun _args -> Nil) in
   stub "run-spec-tests";