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>

spec/tests/test-io-suspension.sx

@@ -207,3 +207,57 @@
         ((final (cek-resume state 41)))
         (assert (cek-terminal? final))
         (assert= (cek-value final) 42)))))
+
+(defsuite
+  "vm-import-suspension"
+  (deftest
+    "vm-execute-module runs trivial bytecode"
+    (let
+      ((globals (dict)) (code (compile-module (quote (42)))))
+      (assert= (vm-execute-module code globals) 42)))
+  (deftest
+    "vm-execute-module converts code-from-value internally"
+    (let
+      ((globals (dict))
+        (code (compile-module (quote ((define x 99) x)))))
+      (assert= (vm-execute-module code globals) 99)))
+  (deftest
+    "compile-module handles import form"
+    (let
+      ((code (compile-module (quote ((import (test lib)))))))
+      (assert (dict? code))
+      (assert (not (nil? (get code :bytecode))))))
+  (deftest
+    "vm-execute-module returns suspension dict on import"
+    (let
+      ((globals (dict))
+        (code (compile-module (quote ((import (test lib)))))))
+      (let
+        ((result (vm-execute-module code globals)))
+        (assert (dict? result) "result should be a dict")
+        (assert= (get result :suspended) true)
+        (assert= (get result :op) "import"))))
+  (deftest
+    "vm-resume-module continues after suspension"
+    (let
+      ((globals (dict))
+        (code
+          (compile-module (quote ((import (test lib)) (define x 42) x)))))
+      (let
+        ((r1 (vm-execute-module code globals)))
+        (assert= (get r1 :suspended) true)
+        (let ((r2 (vm-resume-module r1))) (assert= r2 42)))))
+  (deftest
+    "vm multiple sequential imports suspend and resume"
+    (let
+      ((globals (dict))
+        (code
+          (compile-module
+            (quote ((import (test a)) (import (test b)) (define x 99) x)))))
+      (let
+        ((r1 (vm-execute-module code globals)))
+        (assert= (get r1 :suspended) true)
+        (let
+          ((r2 (vm-resume-module r1)))
+          (assert= (get r2 :suspended) true)
+          (let ((r3 (vm-resume-module r2))) (assert= r3 99)))))))