Post-10d: JIT measurement infrastructure + compiler fixes

Measurement:
- JIT hit/miss/skip counters in sx_runtime.ml (jit_try_call)
- VM instruction counter enabled in run loop
- jit-enable, vm-counters, vm-counters-reset epoch commands
- Test runner --jit flag for opt-in JIT measurement
- Results (132 tests): 5.8% VM hit, 56% evaluator self-calls, 38% anon

Fixes:
- Move compile-provide, compile-scope, compile-guard,
  compile-guard-clauses inside define-library begin block
  (were orphaned outside, causing "Undefined symbol" JIT failures)
- Add deref primitive (signal unwrap with tracking)
- Add deref compiler dispatch
- Fix compile-expr for scope forms to handle non-keyword args

CEK pruning assessment: evaluator self-calls (56%) can't be pruned —
the CEK must evaluate itself. Real pruning requires self-hosting
compiler (Phase 2+). The VM correctly handles user code that
JIT-compiles. 2776/2776 tests pass.

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

hosts/ocaml/bin/run_tests.ml

@@ -1455,17 +1455,68 @@ let run_spec_tests env test_files =
 let () =
   let args = Array.to_list Sys.argv |> List.tl in
   let foundation_only = List.mem "--foundation" args in
+  let jit_enabled = List.mem "--jit" args in
   let test_files = List.filter (fun a -> not (String.length a > 0 && a.[0] = '-')) args in
 
   (* Always run foundation tests *)
   run_foundation_tests ();
 
   if not foundation_only then begin
-    Printf.printf "\n=== SX Spec Tests (CEK Evaluator) ===\n%!";
+    Printf.printf "\n=== SX Spec Tests (CEK Evaluator + JIT) ===\n%!";
     let env = make_test_env () in
+    (* Load compiler and enable JIT (opt-in via --jit flag) *)
+    if jit_enabled then begin
+    let globals = Hashtbl.create 512 in
+    let rec env_to_globals e =
+      Hashtbl.iter (fun id v ->
+        let name = Sx_types.unintern id in
+        if not (Hashtbl.mem globals name) then
+          Hashtbl.replace globals name v) e.Sx_types.bindings;
+      match e.Sx_types.parent with Some p -> env_to_globals p | None -> ()
+    in
+    env_to_globals env;
+    (try
+      let compiler_path = if Sys.file_exists "lib/compiler.sx" then "lib/compiler.sx"
+        else "../../lib/compiler.sx" in
+      let ic = open_in compiler_path in
+      let src = really_input_string ic (in_channel_length ic) in
+      close_in ic; let _ = src in
+      let exprs = Sx_parser.parse_all src in
+      List.iter (fun e -> ignore (Sx_ref.eval_expr e (Env env))) exprs;
+      env_to_globals env;
+      Sx_runtime._jit_try_call_fn := Some (fun f args ->
+        match f with
+        | Lambda l ->
+          (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 _ -> None)
+           | Some _ -> None
+           | None ->
+             if l.l_name = None then None
+             else begin
+               l.l_compiled <- Some Sx_vm.jit_failed_sentinel;
+               match Sx_vm.jit_compile_lambda l globals with
+               | Some cl -> l.l_compiled <- Some cl;
+                 (try Some (Sx_vm.call_closure cl args cl.vm_env_ref) with _ -> None)
+               | None -> None
+             end)
+        | _ -> None);
+      Printf.printf "[jit] Compiler loaded, JIT enabled\n%!"
+    with e ->
+      Printf.printf "[jit] Compiler not loaded: %s\n%!" (Printexc.to_string e));
+    end;
+    Sx_runtime.jit_reset_counters ();
     run_spec_tests env test_files
   end;
 
+  (* JIT statistics *)
+  let jh = !(Sx_runtime._jit_hit) and jm = !(Sx_runtime._jit_miss) and js = !(Sx_runtime._jit_skip) in
+  let total = jh + jm + js in
+  if total > 0 then
+    Printf.printf "\n[jit] calls=%d  hit=%d (%.1f%%)  miss=%d  skip=%d\n"
+      total jh (100.0 *. float_of_int jh /. float_of_int (max 1 total)) jm js;
+
   (* Summary *)
   Printf.printf "\n%s\n" (String.make 60 '=');
   Printf.printf "Results: %d passed, %d failed\n" !pass_count !fail_count;