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>

hosts/ocaml/browser/sx_browser.ml

@@ -237,6 +237,7 @@ let return_via_side_channel (v : Js.Unsafe.any) : Js.Unsafe.any =
    VmClosures from bytecode modules hold vm_env_ref pointing here.
    Must stay in sync so VmClosures see post-boot definitions. *)
 let _vm_globals : (string, value) Hashtbl.t = Hashtbl.create 512
+let () = Sx_types._default_vm_globals := _vm_globals
 let _in_batch = ref false
 
 (* Sync env→VM: copy all bindings from global_env.bindings to _vm_globals.

hosts/ocaml/lib/sx_types.ml

@@ -258,6 +258,10 @@ let _cek_io_resolver : (value -> value -> value) option ref = ref None
     to convert CEK suspensions to VmSuspended for _driveAsync handling. *)
 let _cek_io_suspend_hook : (value -> value) option ref = ref None
 
+(** Default VM globals for stub VMs created during IO suspension.
+    Set by sx_browser.ml to _vm_globals so CEK resume can access platform functions. *)
+let _default_vm_globals : (string, value) Hashtbl.t ref = ref (Hashtbl.create 0)
+
 (** Hook: convert VM suspension exceptions to CekPerformRequest.
     Set by sx_vm after it defines VmSuspended. Called by sx_runtime.sx_apply_cek. *)
 let _convert_vm_suspension : (exn -> unit) ref = ref (fun _ -> ())

hosts/ocaml/lib/sx_vm.ml

@@ -1027,7 +1027,7 @@ let () = _vm_suspension_to_dict := (fun exn ->
    gets swallowed as "IO suspension in non-IO context". *)
 let () = _cek_io_suspend_hook := Some (fun suspended_state ->
   let request = Sx_ref.cek_io_request suspended_state in
-  let vm = create (Hashtbl.create 0) in
+  let vm = create !_default_vm_globals in
   vm.pending_cek <- Some suspended_state;
   raise (VmSuspended (request, vm)))
 

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

shared/static/wasm/sx_browser.bc.wasm.js

@@ -1792,7 +1792,7 @@
             blake2_js_for_wasm_create: blake2_js_for_wasm_create};
    }
    (globalThis))
-({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-5ce001fe",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-8afc1869",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
+({"link":[["runtime-0db9b496",0],["prelude-d7e4b000",0],["stdlib-23ce0836",[]],["re-9a0de245",[2]],["sx-da652b03",[2,3]],["jsoo_runtime-f96b44a8",[2]],["js_of_ocaml-651f6707",[2,5]],["dune__exe__Sx_browser-def18509",[2,4,6]],["std_exit-10fb8830",[2]],["start-f808dbe1",0]],"generated":(b=>{var
 c=b,a=b?.module?.export||b;return{"env":{"caml_ba_kind_of_typed_array":()=>{throw new
 Error("caml_ba_kind_of_typed_array not implemented")},"caml_exn_with_js_backtrace":()=>{throw new
 Error("caml_exn_with_js_backtrace not implemented")},"caml_int64_create_lo_mi_hi":()=>{throw new