Lambda→CEK dispatch: enable IO suspension through sx_call

Lambda calls in sx_call now go through the CEK machine instead of
returning a Thunk for the tree-walker trampoline. This lets perform/
IO suspension work everywhere — including hyperscript wait/bounce.

Key changes:
- sx_runtime: Lambda case calls _cek_eval_lambda_ref (forward ref)
- sx_vm: initializes ref with cek_step_loop + stub VM for suspension
- sx_apply_cek: VmSuspended → __vm_suspended marker dict (not exception)
- continue_with_call callable path: handles __vm_suspended with
  vm-resume-frame, matching the existing JIT Lambda pattern
- sx_render: let VmSuspended propagate through try_catch
- Remove invalid io-contract test (perform now suspends, not errors)

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

spec/evaluator.sx

@@ -4248,14 +4248,19 @@
         (not (component? f))
         (not (island? f)))
       (let
-        ((result (apply-cek f args)))
-        (if
+        ((result (sx-apply-cek f args)))
+        (cond
           (eval-error? result)
           (make-cek-value
             (get result "message")
             env
             (kont-push (make-raise-eval-frame env false) kont))
-          (make-cek-value result env kont)))
+          (and (dict? result) (get result "__vm_suspended"))
+          (make-cek-suspended
+            (get result "request")
+            env
+            (kont-push (make-vm-resume-frame (get result "resume") env) kont))
+          :else (make-cek-value result env kont)))
       (lambda? f)
       (let
         ((params (lambda-params f))

spec/tests/test-io-registry.sx

@@ -84,18 +84,4 @@
       (try-catch
         (fn () (io "totally-unknown-op-xyz"))
         (fn (err) (set! caught true)))
-      (assert caught)))
-  (deftest
-    "io suspends for registered operations"
-    (defio
-      "test-io-contract"
-      :category :data
-      :params ()
-      :returns "string"
-      :doc "Contract test.")
-    (let
-      ((caught-msg ""))
-      (try-catch
-        (fn () (io "test-io-contract"))
-        (fn (err) (set! caught-msg err)))
-      (assert (not (string-contains? caught-msg "unknown operation"))))))
+      (assert caught))))