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/py.sx

@@ -830,11 +830,21 @@
 
 (define py-emit-infix
   (fn ((op :as string) (args :as list) (cell-vars :as list))
-    (let ((py-op (py-op-symbol op)))
-      (if (and (= (len args) 1) (= op "-"))
-        (str "(-" (py-expr-with-cells (first args) cell-vars) ")")
-        (str "(" (py-expr-with-cells (first args) cell-vars)
-             " " py-op " " (py-expr-with-cells (nth args 1) cell-vars) ")")))))
+    (let ((py-op (py-op-symbol op))
+          (n (len args)))
+      (cond
+        (and (= n 1) (= op "-"))
+          (str "(-" (py-expr-with-cells (first args) cell-vars) ")")
+        (= n 2)
+          (str "(" (py-expr-with-cells (first args) cell-vars)
+               " " py-op " " (py-expr-with-cells (nth args 1) cell-vars) ")")
+        ;; Variadic: left-fold (a op b op c op d ...)
+        :else
+          (let ((result (py-expr-with-cells (first args) cell-vars)))
+            (for-each (fn (arg)
+              (set! result (str "(" result " " py-op " " (py-expr-with-cells arg cell-vars) ")")))
+              (rest args))
+            result)))))
 
 
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