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} *)
 

hosts/ocaml/lib/sx_vm_ref.ml

@@ -455,7 +455,20 @@ let () = _vm_call_fn := vm_call
    Public API — matches Sx_vm interface for drop-in replacement
    ================================================================ *)
 
-(** Execute a compiled module — entry point for load-sxbc, compile-blob. *)
+(** Build a suspension dict from __io_request in globals. *)
+let check_io_suspension globals vm_val =
+  match Hashtbl.find_opt globals "__io_request" with
+  | Some req when sx_truthy req ->
+    let d = Hashtbl.create 4 in
+    Hashtbl.replace d "suspended" (Bool true);
+    Hashtbl.replace d "op" (String "import");
+    Hashtbl.replace d "request" req;
+    Hashtbl.replace d "vm" vm_val;
+    Some (Dict d)
+  | _ -> None
+
+(** Execute a compiled module — entry point for load-sxbc, compile-blob.
+    Returns the result value, or a suspension dict if OP_PERFORM fired. *)
 let execute_module (code : vm_code) (globals : (string, value) Hashtbl.t) =
   let cl = { vm_code = code; vm_upvalues = [||]; vm_name = Some "module";
              vm_env_ref = globals; vm_closure_env = None } in
@@ -468,7 +481,25 @@ let execute_module (code : vm_code) (globals : (string, value) Hashtbl.t) =
   done;
   m.vm_frames <- [frame];
   ignore (vm_run vm_val);
-  vm_pop vm_val
+  match check_io_suspension globals vm_val with
+  | Some suspension -> suspension
+  | None -> vm_pop vm_val
+
+(** Resume a suspended module. Clears __io_request, pushes nil, re-runs. *)
+let resume_module (suspended : value) =
+  match suspended with
+  | Dict d ->
+    let vm_val = Hashtbl.find d "vm" in
+    let globals = match vm_val with
+      | VmMachine m -> m.vm_globals
+      | _ -> raise (Eval_error "resume_module: expected VmMachine") in
+    Hashtbl.replace globals "__io_request" Nil;
+    ignore (vm_push vm_val Nil);
+    ignore (vm_run vm_val);
+    (match check_io_suspension globals vm_val with
+     | Some suspension -> suspension
+     | None -> vm_pop vm_val)
+  | _ -> raise (Eval_error "resume_module: expected suspension dict")
 
 (** Execute a closure with args — entry point for JIT Lambda calls. *)
 let call_closure (cl : vm_closure) (args : value list) (globals : (string, value) Hashtbl.t) =