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/lib/sx_primitives.ml

@@ -686,6 +686,11 @@ 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);
+  (* SSR stubs — effect is no-op on server (signals.sx guards with client?),
+     resource returns loading state. Other browser primitives only appear
+     inside effect bodies which never execute during SSR. *)
+  register "effect" (fun _args -> Nil);
+  register "register-in-scope" (fun _args -> Nil);
   (* resource — SSR stub: return signal with {loading: true}, client hydrates real fetch *)
   register "resource" (fun _args ->
     let state = Hashtbl.create 8 in