Step 5: CEK IO suspension + R7RS modules (define-library/import)

Third CEK phase "io-suspended": perform suspends evaluation, host
resolves IO, cek-resume feeds result back. VM OP_PERFORM (opcode 112)
enables JIT-compiled functions to suspend. VM→CEK→suspend chain
propagates suspension across the JIT/CEK boundary via pending_cek.

R7RS define-library creates isolated environments with export control.
import checks the library registry and suspends for unknown libraries,
enabling lazy on-demand loading. Import qualifiers: only, prefix.

Server-side cek_run_with_io handles suspension by dispatching IO
requests to the Python bridge and resuming. guard composes cleanly
with perform for structured error recovery across IO boundaries.

2598/2598 tests (30 new: 15 core suspension, 3 JIT, 1 cross-boundary,
9 modules, 2 error handling). Zero regressions.

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

hosts/ocaml/bootstrap.py

@@ -74,10 +74,13 @@ let () = Sx_primitives._sx_trampoline_fn := !trampoline_fn
 let cek_run_iterative state =
   let s = ref state in
   (try
-    while not (match cek_terminal_p !s with Bool true -> true | _ -> false) do
+    while not (match cek_terminal_p !s with Bool true -> true | _ -> false)
+       && not (match cek_suspended_p !s with Bool true -> true | _ -> false) do
       s := cek_step !s
     done;
-    cek_value !s
+    (match cek_suspended_p !s with
+     | Bool true -> raise (Eval_error "IO suspension in non-IO context")
+     | _ -> cek_value !s)
   with Eval_error msg ->
     _last_error_kont_ref := cek_kont !s;
     raise (Eval_error msg))