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/lib/sx_runtime.ml

@@ -470,13 +470,17 @@ let mutable_list () = ListRef (ref [])
 (* JIT try-call — ref set by sx_server.ml after compiler loads.
    Returns Nil (no JIT) or the result value. Spec calls this. *)
 let _jit_try_call_fn : (value -> value list -> value option) option ref = ref None
+let _jit_hit = ref 0
+let _jit_miss = ref 0
+let _jit_skip = ref 0
+let jit_reset_counters () = _jit_hit := 0; _jit_miss := 0; _jit_skip := 0
 let jit_try_call f args =
   match !_jit_try_call_fn with
-  | None -> Nil
+  | None -> incr _jit_skip; Nil
   | Some hook ->
     match f with
     | Lambda l when l.l_name <> None ->
       let arg_list = match args with List a | ListRef { contents = a } -> a | _ -> [] in
-      (match hook f arg_list with Some result -> result | None -> Nil)
-    | _ -> Nil
+      (match hook f arg_list with Some result -> incr _jit_hit; result | None -> incr _jit_miss; Nil)
+    | _ -> incr _jit_skip; Nil