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/js.sx

@@ -1259,11 +1259,21 @@
 
 (define js-emit-infix
   (fn ((op :as string) (args :as list))
-    (let ((js-op (js-op-symbol op)))
-      (if (and (= (len args) 1) (= op "-"))
-        (str "(-" (js-expr (first args)) ")")
-        (str "(" (js-expr (first args))
-             " " js-op " " (js-expr (nth args 1)) ")")))))
+    (let ((js-op (js-op-symbol op))
+          (n (len args)))
+      (cond
+        (and (= n 1) (= op "-"))
+          (str "(-" (js-expr (first args)) ")")
+        (= n 2)
+          (str "(" (js-expr (first args))
+               " " js-op " " (js-expr (nth args 1)) ")")
+        ;; Variadic: left-fold (a op b op c op d ...)
+        :else
+          (let ((result (js-expr (first args))))
+            (for-each (fn (arg)
+              (set! result (str "(" result " " js-op " " (js-expr arg) ")")))
+              (rest args))
+            result)))))
 
 
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