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>

hosts/ocaml/lib/sx_vm.ml

@@ -598,6 +598,16 @@ let execute_module code globals =
   run vm;
   pop vm
 
+(** Execute module, catching VmSuspended locally (same compilation unit).
+    Returns [Ok result] or [Error (request, vm)] for import suspension.
+    Needed because js_of_ocaml can't catch exceptions across module boundaries. *)
+let execute_module_safe code globals =
+  try
+    let result = execute_module code globals in
+    Ok result
+  with VmSuspended (request, vm) ->
+    Error (request, vm)
+
 
 (** {1 Lazy JIT compilation} *)