A defisland that lets users type an SX expression, step through CEK
evaluation one transition at a time, and see C/E/K registers update
live. Demonstrates that cek-step is pure data->data.
- cek.sx geography: add ~geography/cek/demo-stepper island with
source input, step/run/reset buttons, state display, step history
- platform_js.py: register CEK stepping primitives (make-cek-state,
cek-step, cek-terminal?, cek-value, make-env, sx-serialize) so
island code can access them
Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>