Add Freeze/Thaw page under CEK Machine with live demo

Documents and demonstrates serializable CEK state. Type an expression,
step to any point, click Freeze to see the frozen SX. Click Thaw to
resume from the frozen state and get the result.

- New page at /sx/(geography.(cek.freeze))
- Nav entry under CEK Machine
- Interactive island demo with step/run/freeze/thaw buttons
- Documentation: the idea, freeze format, thaw/resume, what it enables

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

sx/sx/nav-data.sx

@@ -174,7 +174,9 @@
   (dict :label "Overview" :href "/sx/(geography.(cek))"
         :summary "The CEK machine — explicit evaluator with Control, Environment, Kontinuation. Three registers, pure step function.")
   (dict :label "Demo" :href "/sx/(geography.(cek.demo))"
-        :summary "Live islands evaluated by the CEK machine. Counter, computed chains, reactive attributes — all through explicit continuation frames.")))
+        :summary "Live islands evaluated by the CEK machine. Counter, computed chains, reactive attributes — all through explicit continuation frames.")
+  (dict :label "Freeze / Thaw" :href "/sx/(geography.(cek.freeze))"
+        :summary "Serialize a CEK state to s-expressions. Ship it, store it, content-address it. Thaw and resume anywhere.")))
 
 (define plans-nav-items (list
   (dict :label "Status" :href "/sx/(etc.(plan.status))"