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_browser.ml

@@ -555,7 +555,62 @@ let () =
   bind "request-arg" (fun args -> match args with [_; d] -> d | _ -> Nil);
   bind "request-method" (fun _ -> String "GET");
   bind "ctx" (fun _ -> Nil);
-  bind "helper" (fun _ -> Nil)
+  bind "helper" (fun _ -> Nil);
+  ()
+
+(* ================================================================== *)
+(* JIT compilation hook                                                *)
+(*                                                                     *)
+(* On first call to a named lambda, try to compile it to bytecode via  *)
+(* compiler.sx (loaded as an .sx platform file). Compiled closures run *)
+(* on the bytecode VM; failures fall back to the CEK interpreter.      *)
+(* ================================================================== *)
+
+let _jit_compiling = ref false
+let _jit_enabled = ref false
+
+let () =
+  (* Convert int-keyed env.bindings to string-keyed Hashtbl for VM globals *)
+  let env_to_vm_globals env =
+    let g = Hashtbl.create (Hashtbl.length env.bindings) in
+    Hashtbl.iter (fun id v -> Hashtbl.replace g (unintern id) v) env.bindings;
+    g
+  in
+  Sx_ref.jit_call_hook := Some (fun f args ->
+    match f with
+    | Lambda l when !_jit_enabled ->
+      (match l.l_compiled with
+       | Some cl when not (Sx_vm.is_jit_failed cl) ->
+         (try Some (Sx_vm.call_closure cl args cl.vm_env_ref)
+          with e ->
+            let fn_name = match l.l_name with Some n -> n | None -> "?" in
+            Printf.eprintf "[jit] DISABLED %s — %s\n%!" fn_name (Printexc.to_string e);
+            l.l_compiled <- Some Sx_vm.jit_failed_sentinel;
+            None)
+       | Some _ -> None
+       | None ->
+         if !_jit_compiling then None
+         else begin
+           _jit_compiling := true;
+           let globals = env_to_vm_globals global_env in
+           let compiled = Sx_vm.jit_compile_lambda l globals in
+           _jit_compiling := false;
+           let fn_name = match l.l_name with Some n -> n | None -> "?" in
+           (match compiled with
+           | Some cl ->
+             l.l_compiled <- Some cl;
+             (try Some (Sx_vm.call_closure cl args cl.vm_env_ref)
+              with e ->
+                Printf.eprintf "[jit] DISABLED %s — %s\n%!" fn_name (Printexc.to_string e);
+                l.l_compiled <- Some Sx_vm.jit_failed_sentinel;
+                None)
+           | None ->
+             Printf.eprintf "[jit] FAIL %s\n%!" fn_name;
+             None)
+         end)
+    | _ -> None)
+
+let () = ignore (env_bind global_env "enable-jit!" (NativeFn ("enable-jit!", fun _ -> _jit_enabled := true; Nil)))
 
 (* ================================================================== *)
 (* Register global SxKernel object                                     *)