Fix event-bridge + add client? primitive + header store foundation

- Event-bridge: rewrite island to use document-level addEventListener
  via effect + host-callback, bypassing broken container-ref + schedule-idle.
  Also use host-get for event-detail (WASM host handles).

- Add client? primitive: false on server (sx_primitives._is_client ref),
  true in browser (sx_browser.ml sets ref). Enables SSR-safe conditional
  logic for client-only features like def-store.

- Header island: use def-store for idx/shade signals when client? is true,
  falling back to plain signals on server. Foundation for SPA nav state
  preservation (store registry persistence still needs work).

- Remove unused client? K.eval override from sx-platform.js.

100 passed, 1 skipped (isomorphic nav — store registry resets on SPA nav), 0 failed.

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

hosts/ocaml/bin/sx_server.ml

@@ -657,6 +657,8 @@ let make_server_env () =
     match v with
     | Thunk (body, closure_env) -> Sx_ref.eval_expr body (Env closure_env)
     | other -> other);
+  (* client? returns false on server — overridden in browser via K.eval *)
+  ignore (env_bind env "client?" (NativeFn ("client?", fun _ -> Bool false)));
   env
 
 

hosts/ocaml/browser/sx_browser.ml

@@ -439,6 +439,9 @@ let api_fn_arity fn_js =
 let () =
   let bind name fn = ignore (env_bind global_env name (NativeFn (name, fn))) in
 
+  (* client? returns true in browser — set the ref so the primitive returns true *)
+  Sx_primitives._is_client := true;
+
   (* --- Evaluation --- *)
   bind "cek-eval" (fun args ->
     match args with

hosts/ocaml/lib/sx_primitives.ml

@@ -12,6 +12,7 @@ let _sx_call_fn : (value -> value list -> value) ref =
   ref (fun _ _ -> raise (Eval_error "sx_call not initialized"))
 let _sx_trampoline_fn : (value -> value) ref =
   ref (fun v -> v)
+let _is_client : bool ref = ref false
 
 let register name fn = Hashtbl.replace primitives name fn
 
@@ -664,6 +665,8 @@ let () =
     match args with [String msg] -> raise (Eval_error msg)
     | [a] -> raise (Eval_error (to_string a))
     | _ -> raise (Eval_error "error: 1 arg"));
+  (* client? — false by default (server); sx_browser.ml sets _is_client := true *)
+  register "client?" (fun _args -> Bool !_is_client);
   register "apply" (fun args ->
     let call f a =
       match f with