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/platform_js.py

@@ -983,6 +983,7 @@ PRIMITIVES_JS_MODULES: dict[str, str] = {
   PRIMITIVES["ends-with?"] = function(s, p) { var str = String(s); return str.indexOf(p, str.length - p.length) !== -1; };
   PRIMITIVES["slice"] = function(c, a, b) { if (!c || typeof c.slice !== "function") { console.error("[sx-debug] slice called on non-sliceable:", typeof c, c, "a=", a, "b=", b, new Error().stack); return []; } return b !== undefined ? c.slice(a, b) : c.slice(a); };
   PRIMITIVES["substring"] = function(s, a, b) { return String(s).substring(a, b); };
+  PRIMITIVES["char-from-code"] = function(n) { return String.fromCharCode(n); };
   PRIMITIVES["string-length"] = function(s) { return String(s).length; };
   PRIMITIVES["string-contains?"] = function(s, sub) { return String(s).indexOf(String(sub)) !== -1; };
   PRIMITIVES["concat"] = function() {
@@ -1600,6 +1601,7 @@ PLATFORM_PARSER_JS = r"""
     return s.replace(/\\/g, "\\\\").replace(/"/g, '\\"').replace(/\n/g, "\\n").replace(/\t/g, "\\t");
   }
   function sxExprSource(e) { return typeof e === "string" ? e : String(e); }
+  var charFromCode = PRIMITIVES["char-from-code"];
 """