Fix transpiler append! emit for mutable globals + run_with_io error recovery

The transpiler's append! emit path didn't check ml-is-mutable-global?,
so (append! *provide-batch-queue* sub) wrote to a dead local variable
instead of the global _ref. This caused the combined test suite hang —
fire-provide-subscribers was silently broken before the local-ref shadow
removal, and now correctly modifies the global batch queue.

Also adds run_with_io error-to-raise conversion (kont_has_handler guard)
so native Eval_errors can be caught by CEK guard/handler-bind when running
through the test runner's IO-aware step loop.

2798/2800 tests pass. 2 foreign-type-checking failures remain: guard can't
catch Eval_error from native fns called through cek_run_iterative (the
handler dispatch itself uses cek_call which re-enters cek_run_iterative,
creating an infinite loop). Fix requires spec-level change: make (error)
use CEK raise instead of host-error.

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

hosts/ocaml/bin/run_tests.ml

@@ -1231,9 +1231,32 @@ let run_spec_tests env test_files =
     let s = ref state in
     let is_terminal st = match Sx_ref.cek_terminal_p st with Bool true -> true | _ -> false in
     let is_suspended st = match Sx_runtime.get_val st (String "phase") with String "io-suspended" -> true | _ -> false in
+    (* Check if kont has any handler frames — pure structural scan *)
+    let kont_has_handler kont =
+      let k = ref kont in
+      let found = ref false in
+      while (match !k with List (_::_) -> true | _ -> false) && not !found do
+        (match !k with
+         | List (frame :: rest) ->
+           (match frame with
+            | CekFrame f when f.cf_type = "handler" -> found := true
+            | _ -> ());
+           k := List rest
+         | _ -> k := List [])
+      done;
+      !found in
     let rec loop () =
       while not (is_terminal !s) && not (is_suspended !s) do
-        s := Sx_ref.cek_step !s
+        (try s := Sx_ref.cek_step !s
+         with Eval_error msg ->
+           let kont = Sx_ref.cek_kont !s in
+           if kont_has_handler kont then
+             (* Convert to CEK-level raise so guard/handler-bind can catch it *)
+             let env = Sx_ref.cek_env !s in
+             s := Sx_ref.make_cek_value (String msg) env
+                    (Sx_ref.kont_push (Sx_ref.make_raise_eval_frame env (Bool false)) kont)
+           else
+             raise (Eval_error msg))
       done;
       if is_suspended !s then begin
         let request = Sx_runtime.get_val !s (String "request") in