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/page-functions.sx

@@ -528,4 +528,6 @@
         "spec-explorer" '(~plans/spec-explorer/plan-spec-explorer-content)
         "sx-urls" '(~plans/sx-urls/plan-sx-urls-content)
         "sx-protocol" '(~plans/sx-protocol/plan-sx-protocol-content)
+        "scoped-effects" '(~plans/scoped-effects/plan-scoped-effects-content)
+        "foundations" '(~plans/foundations/plan-foundations-content)
         :else '(~plans/index/plans-index-content)))))