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>

shared/sx/ref/primitives.sx

@@ -323,6 +323,11 @@
   :returns "number"
   :doc "Length of string in characters.")
 
+(define-primitive "char-from-code"
+  :params ((n :as number))
+  :returns "string"
+  :doc "Convert Unicode code point to single-character string.")
+
 (define-primitive "substring"
   :params ((s :as string) (start :as number) (end :as number))
   :returns "string"