Unicode escapes, variadic infix fix, spreads demos, scoped-effects + foundations plans

- Add \uXXXX unicode escape support to parser.py and parser.sx spec
- Add char-from-code primitive (Python chr(), JS String.fromCharCode())
- Fix variadic infix operators in both bootstrappers (js.sx, py.sx) —
  (+ a b c d) was silently dropping terms, now left-folds correctly
- Rebootstrap sx_ref.py and sx-browser.js with all fixes
- Fix 3 pre-existing map-dict test failures in shared/sx/tests/run.py
- Add live demos alongside examples in spreads essay (side-by-side layout)
- Add scoped-effects plan: algebraic effects as unified foundation for
  spread/collect/island/lake/signal/context
- Add foundations plan: CEK machine, the computational floor, three-axis
  model (depth/topology/linearity), Curry-Howard correspondence
- Route both plans in page-functions.sx and nav-data.sx

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

sx/sx/nav-data.sx

@@ -220,7 +220,11 @@
   (dict :label "Spec Explorer" :href "/sx/(etc.(plan.spec-explorer))"
         :summary "The fifth ring — SX exploring itself. Per-function cards showing source, Python/JS/Z3 translations, platform dependencies, tests, proofs, and usage examples.")
   (dict :label "SX Protocol" :href "/sx/(etc.(plan.sx-protocol))"
-        :summary "S-expressions as a universal protocol for networked hypermedia — replacing URLs, HTTP verbs, query languages, and rendering with one evaluable format.")))
+        :summary "S-expressions as a universal protocol for networked hypermedia — replacing URLs, HTTP verbs, query languages, and rendering with one evaluable format.")
+  (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).")))
 
 (define reactive-islands-nav-items (list
   (dict :label "Overview" :href "/sx/(geography.(reactive))"