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>