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>

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-64e6b16e",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-2ac146e9",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-951e6734",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-6b156118",[2,3,5]],["std_exit-10fb8830",[2]],["start-80fdb768",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new