Browser JIT: compile SX lambdas to bytecode VM in WASM kernel

- Wire up jit_call_hook in sx_browser.ml (same pattern as server)
- Deferred JIT: _jit_enabled flag, enabled after boot-init completes
  (prevents "Undefined symbol" errors from compiling during .sx loading)
- enable-jit! native function called by sx-platform.js after boot
- sx-platform.js: async WASM kernel polling + JIT enable after init
- Error logging for JIT compile failures and runtime fallbacks

Performance: 858ms → 431ms (WASM CEK) → 101ms (WASM JIT)

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

shared/static/wasm/sx-platform.js

@@ -339,6 +339,7 @@
     var _doInit = function() {
       loadWebStack();
       Sx.init();
+      K.eval('(enable-jit!)');
     };
 
     if (document.readyState === "loading") {

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-ea36a0db",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-4feaf1fd",[2,3,5]],["std_exit-10fb8830",[2]],["start-9e868ccd",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["sx-ea36a0db",[2]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,4]],["dune__exe__Sx_browser-cafa954c",[2,3,5]],["std_exit-10fb8830",[2]],["start-29cf9a72",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