geography/cek.sx: overview page (three registers, deref-as-shift explanation) + demo page with 5 live islands (counter, computed chain, reactive attrs, stopwatch effect+cleanup, batch coalescing). Nav entry, router routes, defpage definitions. CEK exports (cekRun, makeCekState, makeReactiveResetFrame, evalExpr) added to Sx public API via platform_js.py. Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>