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>

hosts/ocaml/browser/sx-platform.js

@@ -16,8 +16,7 @@
 (function() {
   "use strict";
 
-  var K = globalThis.SxKernel;
-  if (!K) { console.error("[sx-platform] SxKernel not found"); return; }
+  function boot(K) {
 
   // ================================================================
   // 8 FFI Host Primitives
@@ -340,6 +339,7 @@
     var _doInit = function() {
       loadWebStack();
       Sx.init();
+      K.eval('(enable-jit!)');
     };
 
     if (document.readyState === "loading") {
@@ -349,4 +349,16 @@
     }
   }
 
+  } // end boot
+
+  // SxKernel is available synchronously (js_of_ocaml) or after async
+  // WASM init. Poll briefly to handle both cases.
+  var K = globalThis.SxKernel;
+  if (K) { boot(K); return; }
+  var tries = 0;
+  var poll = setInterval(function() {
+    K = globalThis.SxKernel;
+    if (K) { clearInterval(poll); boot(K); }
+    else if (++tries > 100) { clearInterval(poll); console.error("[sx-platform] SxKernel not found after 5s"); }
+  }, 50);
 })();