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_vm.ml

@@ -204,6 +204,7 @@ let jit_compile_comp ~name ~params ~has_children ~body ~closure globals =
     Returns the result value, or raises VmSuspended if CEK suspends.
     Saves the suspended CEK state in vm.pending_cek for later resume. *)
 let cek_call_or_suspend vm f args =
+  incr _vm_cek_count;
   let a = match args with Nil -> [] | List l -> l | _ -> [args] in
   let state = Sx_ref.continue_with_call f (List a) (Env (Sx_types.make_env ())) (List a) (List []) in
   let final = Sx_ref.cek_step_loop state in
@@ -358,6 +359,7 @@ and run vm =
         let saved_ip = frame.ip in
         let op = bc.(frame.ip) in
         frame.ip <- frame.ip + 1;
+        incr _vm_insn_count;
         (try match op with
         (* ---- Constants ---- *)
         | 1 (* OP_CONST *) ->