Add CEK reactive tests (9/9), fix test runners for CEK-default mode

test-cek-reactive.sx: 9 tests across 4 suites — deref pass-through,
signal without reactive-reset, reactive-reset shift with continuation
capture, scope disposal cleanup. run_cek_reactive_tests.py: new runner
loading signals+frames+cek. Both test runners override sx_ref.eval_expr
back to tree-walk so interpreted .sx uses tree-walk internally.
Plan page added to sx-docs.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/nav-data.sx

@@ -226,7 +226,9 @@
   (dict :label "Scoped Effects" :href "/sx/(etc.(plan.scoped-effects))"
         :summary "Algebraic effects as the unified foundation — spreads, islands, lakes, signals, and context are all instances of one primitive: a named scope with downward value, upward accumulation, and a propagation mode.")
   (dict :label "Foundations" :href "/sx/(etc.(plan.foundations))"
-        :summary "The computational floor — from scoped effects through algebraic effects and delimited continuations to the CEK machine. Why three registers are irreducible, and the three-axis model (depth, topology, linearity).")))
+        :summary "The computational floor — from scoped effects through algebraic effects and delimited continuations to the CEK machine. Why three registers are irreducible, and the three-axis model (depth, topology, linearity).")
+  (dict :label "Deref as Shift" :href "/sx/(etc.(plan.cek-reactive))"
+        :summary "Phase B: replace explicit effect wrapping with implicit continuation capture. Deref inside reactive-reset performs shift, capturing the rest of the expression as the subscriber.")))
 
 (define reactive-islands-nav-items (list
   (dict :label "Overview" :href "/sx/(geography.(reactive))"