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_primitives.ml

@@ -1393,6 +1393,20 @@ let () =
       end else Nil
     | _ -> Nil);
 
+  (* deref — unwrap a signal value with reactive dependency tracking.
+     If value is a Signal, returns s_value and registers in tracking context.
+     Otherwise returns value as-is. *)
+  register "deref" (fun args ->
+    match args with
+    | [Signal sig'] ->
+      if !_tracking_active then begin
+        if not (List.memq (Signal sig') !_tracking_deps) then
+          _tracking_deps := Signal sig' :: !_tracking_deps
+      end;
+      sig'.s_value
+    | [v] -> v
+    | _ -> Nil);
+
   (* bind — create a tracked computation. Takes a body-fn (lambda).
      Starts tracking, evaluates body, collects deps, subscribes.
      On dep change: unsubscribes, re-evaluates, re-subscribes.