Stub VM uses real globals for CEK resume after IO suspension

The _cek_io_suspend_hook creates a stub VM to carry the suspended CEK
state. Previously used empty globals, which caused "Not callable: nil"
when the CEK resume needed platform functions. Now uses _default_vm_globals
(set to _vm_globals by sx_browser.ml) so all platform functions and
definitions are available during resume.

Remaining issue: still getting "resume: Not callable: nil" — the CEK
continuation env may not include letrec bindings from the island body.
The suspension point is inside reload-frame → hs-wait, and the resume
needs to call wait-boot (a letrec binding).

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

shared/static/wasm/sx/hs-compiler.sx

@@ -1035,7 +1035,15 @@
                   ((fn-expr (hs-to-sx (nth ast 1)))
                     (args (map hs-to-sx (nth ast 2))))
                   (cons fn-expr args)))
-              ((= head (quote return)) (hs-to-sx (nth ast 1)))
+              ((= head (quote return))
+                (let
+                  ((val (nth ast 1)))
+                  (if
+                    (nil? val)
+                    (list (quote raise) (list (quote list) "hs-return" nil))
+                    (list
+                      (quote raise)
+                      (list (quote list) "hs-return" (hs-to-sx val))))))
               ((= head (quote throw))
                 (list (quote raise) (hs-to-sx (nth ast 1))))
               ((= head (quote settle))
@@ -1106,13 +1114,41 @@
                   (quote hs-init)
                   (list (quote fn) (list) (hs-to-sx (nth ast 1)))))
               ((= head (quote def))
-                (list
-                  (quote define)
-                  (make-symbol (nth ast 1))
+                (let
+                  ((body (hs-to-sx (nth ast 3)))
+                    (params
+                      (map
+                        (fn
+                          (p)
+                          (if
+                            (and (list? p) (= (first p) (quote ref)))
+                            (make-symbol (nth p 1))
+                            (make-symbol p)))
+                        (nth ast 2))))
                   (list
-                    (quote fn)
-                    (map make-symbol (nth ast 2))
-                    (hs-to-sx (nth ast 3)))))
+                    (quote define)
+                    (make-symbol (nth ast 1))
+                    (list
+                      (quote fn)
+                      params
+                      (list
+                        (quote guard)
+                        (list
+                          (quote _e)
+                          (list
+                            (quote true)
+                            (list
+                              (quote if)
+                              (list
+                                (quote and)
+                                (list (quote list?) (quote _e))
+                                (list
+                                  (quote =)
+                                  (list (quote first) (quote _e))
+                                  "hs-return"))
+                              (list (quote nth) (quote _e) 1)
+                              (list (quote raise) (quote _e)))))
+                        body)))))
               ((= head (quote behavior)) (emit-behavior ast))
               ((= head (quote sx-eval))
                 (let