Fix JIT mutable closure bug: vm-global-get now checks closure env first

vm-global-get checked vm.globals before closure-env, while vm-global-set
wrote to closure-env first. This asymmetry meant set! mutations to mutable
closure variables (e.g. parser position counters) were invisible to sibling
closures reading via JIT — they saw stale snapshots in the globals table.

Reversed vm-global-get lookup order: closure env → globals → primitives,
matching vm-global-set. Also enabled JIT in the MCP harness (compiler.sx
loading, env_bind hook for live globals sync, jit_try_call hook) so
sx_harness_eval exercises the same code path as the server.

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

lib/vm.sx

@@ -297,27 +297,32 @@
       vm-global-get
       (fn
         (vm frame name)
-        "Look up a global: globals table → closure env → primitives → HO wrappers"
+        "Look up a global: closure env → globals table → primitives → HO forms"
         (let
-          ((globals (vm-globals-ref vm)))
+          ((closure-env (get (frame-closure frame) "vm-closure-env")))
           (if
-            (has-key? globals name)
-            (get globals name)
+            (nil? closure-env)
             (let
-              ((closure-env (-> frame frame-closure closure-env)))
+              ((globals (vm-globals-ref vm)))
               (if
-                (nil? closure-env)
+                (has-key? globals name)
+                (get globals name)
                 (cek-try
                   (fn () (get-primitive name))
-                  (fn (e) (vm-resolve-ho-form vm name)))
+                  (fn (e) (vm-resolve-ho-form vm name)))))
+            (let
+              ((found (env-walk closure-env name)))
+              (if
+                (nil? found)
                 (let
-                  ((found (env-walk closure-env name)))
+                  ((globals (vm-globals-ref vm)))
                   (if
-                    (nil? found)
+                    (has-key? globals name)
+                    (get globals name)
                     (cek-try
                       (fn () (get-primitive name))
-                      (fn (e) (vm-resolve-ho-form vm name)))
-                    found))))))))
+                      (fn (e) (vm-resolve-ho-form vm name)))))
+                found))))))
     (define
       vm-resolve-ho-form
       (fn