CEK-native higher-order forms: map, filter, reduce, some, every?, for-each

Higher-order forms now step element-by-element through the CEK machine
using dedicated frames instead of delegating to tree-walk ho-map etc.
Each callback invocation goes through continue-with-call, so deref-as-shift
works inside map/filter/reduce callbacks in reactive island contexts.

- cek.sx: rewrite step-ho-* to use CEK frames, add frame handlers in
  step-continue for map, filter, reduce, for-each, some, every
- frames.sx: add SomeFrame, EveryFrame, MapIndexedFrame
- test-cek-reactive.sx: add 10 tests for CEK-native HO forms

89 tests pass (20 signal + 43 CEK + 26 CEK reactive).

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

shared/sx/ref/frames.sx

@@ -112,10 +112,14 @@
   (fn (remaining env)
     {:type "thread" :remaining remaining :env env}))
 
-;; MapFrame: higher-order map in progress
+;; MapFrame: higher-order map/map-indexed in progress
 (define make-map-frame
   (fn (f remaining results env)
-    {:type "map" :f f :remaining remaining :results results :env env}))
+    {:type "map" :f f :remaining remaining :results results :env env :indexed false}))
+
+(define make-map-indexed-frame
+  (fn (f remaining results env)
+    {:type "map" :f f :remaining remaining :results results :env env :indexed true}))
 
 ;; FilterFrame: higher-order filter in progress
 (define make-filter-frame
@@ -133,6 +137,16 @@
   (fn (f remaining env)
     {:type "for-each" :f f :remaining remaining :env env}))
 
+;; SomeFrame: higher-order some (short-circuit on first truthy)
+(define make-some-frame
+  (fn (f remaining env)
+    {:type "some" :f f :remaining remaining :env env}))
+
+;; EveryFrame: higher-order every? (short-circuit on first falsy)
+(define make-every-frame
+  (fn (f remaining env)
+    {:type "every" :f f :remaining remaining :env env}))
+
 ;; ScopeFrame: scope-pop! when frame pops
 (define make-scope-frame
   (fn (name remaining env)