Fix 2 pre-existing scope test failures: CEK-to-scope_stacks fallback

When aser manages scope via scope_stacks but a sub-expression falls
through to the CEK machine, context/emit!/emitted couldn't find the
scope frames (they're in scope_stacks, not on the kont). Now the CEK
special forms fall back to env-bound primitives when kont lookup fails.

2568/2568 tests pass (was 2566/2568).

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

spec/evaluator.sx

@@ -1701,7 +1701,11 @@
             nil))
         (frame (kont-find-provide kont name)))
       (make-cek-value
-        (if (nil? frame) default-val (get frame "value"))
+        (if frame
+          (get frame "value")
+          (if (env-has? env "context")
+            (apply (env-get env "context") (list name default-val))
+            default-val))
         env
         kont))))
 
@@ -1713,13 +1717,14 @@
       ((name (trampoline (eval-expr (first args) env)))
         (val (trampoline (eval-expr (nth args 1) env)))
         (frame (kont-find-scope-acc kont name)))
-      (when
-        frame
-        (dict-set!
-          frame
-          "emitted"
-          (append (get frame "emitted") (list val))))
-      (make-cek-value nil env kont))))
+      (if frame
+        (do
+          (dict-set! frame "emitted" (append (get frame "emitted") (list val)))
+          (make-cek-value nil env kont))
+        (do
+          (when (env-has? env "scope-emit!")
+            (apply (env-get env "scope-emit!") (list name val)))
+          (make-cek-value nil env kont))))))
 
 (define
   step-sf-emitted
@@ -1729,7 +1734,11 @@
       ((name (trampoline (eval-expr (first args) env)))
         (frame (kont-find-scope-acc kont name)))
       (make-cek-value
-        (if (nil? frame) (list) (get frame "emitted"))
+        (if frame
+          (get frame "emitted")
+          (if (env-has? env "emitted")
+            (apply (env-get env "emitted") (list name))
+            (list)))
         env
         kont))))