VM import suspension for browser lazy loading

Bytecode compiler now emits OP_PERFORM for (import ...) and compiles
(define-library ...) bodies. The VM stores the import request in
globals["__io_request"] and stops the run loop — no exceptions needed.
vm-execute-module returns a suspension dict, vm-resume-module continues.

Browser: sx_browser.ml detects suspension dicts from execute_module and
returns JS {suspended, op, request, resume} objects. The sx-platform.js
while loop handles cascading suspensions via handleImportSuspension.

13 modules load via .sxbc bytecode in 226ms (manifest-driven), both
islands hydrate, all handlers wired. 2650/2650 tests pass including
6 new vm-import-suspension tests.

Also: consolidated sx-platform-2.js → sx-platform.js, fixed
vm-execute-module missing code-from-value call, fixed bootstrap.py
protocol registry transpiler issues.

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

lib/vm.sx

@@ -62,7 +62,8 @@
     vm-run
     vm-step
     vm-call-closure
-    vm-execute-module)
+    vm-execute-module
+    vm-resume-module)
   (begin
     (define make-upvalue-cell (fn (value) {:uv-value value}))
     (define uv-get (fn (cell) (get cell "uv-value")))
@@ -443,7 +444,11 @@
                   (if
                     (>= (frame-ip frame) (len bc))
                     (vm-set-frames! vm (list))
-                    (do (vm-step vm frame rest-frames bc consts) (loop))))))))
+                    (do
+                      (vm-step vm frame rest-frames bc consts)
+                      (when
+                        (nil? (get (vm-globals-ref vm) "__io_request"))
+                        (loop)))))))))
         (loop)))
     (define
       vm-step
@@ -595,8 +600,7 @@
             (= op 112)
             (let
               ((request (vm-pop vm)))
-              (error
-                (str "VM: IO suspension (OP_PERFORM) — request: " request)))
+              (dict-set! (vm-globals-ref vm) "__io_request" request))
             :else (error (str "VM: unknown opcode " op))))))
     (define
       vm-call-closure
@@ -614,14 +618,29 @@
       (fn
         (code globals)
         (let
-          ((closure (make-vm-closure code (list) "module" globals nil))
-            (vm (make-vm globals)))
+          ((vm-code (code-from-value code)) (vm (make-vm globals)))
           (let
-            ((frame (make-vm-frame closure 0)))
-            (pad-n-nils vm (code-locals code))
+            ((closure (make-vm-closure vm-code (list) "module" globals nil))
+              (frame (make-vm-frame closure 0)))
+            (pad-n-nils vm (code-locals vm-code))
             (vm-set-frames! vm (list frame))
             (vm-run vm)
-            (vm-pop vm))))))) ;; end define-library
+            (let
+              ((io-req (get (vm-globals-ref vm) "__io_request")))
+              (if (nil? io-req) (vm-pop vm) {:vm vm :suspended true :op "import" :request io-req}))))))
+    (define
+      vm-resume-module
+      (fn
+        (suspended-result)
+        "Resume a suspended VM after IO (import) has been resolved.\n     Clears __io_request in globals, pushes nil (import result), re-runs."
+        (let
+          ((vm (get suspended-result :vm)))
+          (dict-set! (vm-globals-ref vm) "__io_request" nil)
+          (vm-push vm nil)
+          (vm-run vm)
+          (let
+            ((io-req (get (vm-globals-ref vm) "__io_request")))
+            (if (nil? io-req) (vm-pop vm) {:vm vm :suspended true :op "import" :request io-req}))))))) ;; end define-library
 
 ;; Re-export to global namespace for backward compatibility
 (import (sx vm))