Fix env-shadowing: rebind host extension points after .sx file load

evaluator.sx defines *custom-special-forms* and register-special-form!
which shadow the host's native bindings when loaded at runtime. The
native bindings route to Sx_ref.custom_special_forms (the dict the CEK
evaluator checks), but the SX-level defines create a separate dict.

Fix: rebind_host_extensions runs after every load command, re-asserting
the native register-special-form! and *custom-special-forms* bindings.

Add regression test: custom form registered before evaluator.sx load
survives and remains callable via CEK dispatch afterward.

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

hosts/ocaml/bin/sx_server.ml

@@ -711,6 +711,18 @@ let register_jit_hook env =
 
 
 (* ====================================================================== *)
+(* Re-assert host-provided extension points after loading .sx files.
+   evaluator.sx defines *custom-special-forms* and register-special-form!
+   which shadow the native bindings from setup_evaluator_bridge. *)
+let rebind_host_extensions env =
+  Hashtbl.replace env.bindings "register-special-form!"
+    (NativeFn ("register-special-form!", fun args ->
+      match args with
+      | [String name; handler] ->
+        ignore (Sx_ref.register_special_form (String name) handler); Nil
+      | _ -> raise (Eval_error "register-special-form!: expected (name handler)")));
+  ignore (env_bind env "*custom-special-forms*" Sx_ref.custom_special_forms)
+
 (* Command dispatch                                                        *)
 (* ====================================================================== *)
 
@@ -727,6 +739,9 @@ let rec dispatch env cmd =
         ignore (Sx_ref.eval_expr expr (Env env));
         incr count
       ) exprs;
+      (* Rebind host extension points after .sx load — evaluator.sx
+         defines *custom-special-forms* which shadows the native dict *)
+      rebind_host_extensions env;
       send_ok_value (Number (float_of_int !count))
     with
     | Eval_error msg -> send_error msg
@@ -1142,7 +1157,9 @@ let cli_load_files env files =
         ignore (Sx_ref.eval_expr expr (Env env))
       ) exprs
     end
-  ) files
+  ) files;
+  (* Rebind after load in case .sx files shadowed host extension points *)
+  rebind_host_extensions env
 
 let cli_mode mode =
   let env = make_server_env () in