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/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