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/tests/run.py

@@ -940,6 +940,18 @@ def _load_types(env):
     # types.sx uses component-has-children (no ?), test runner has component-has-children?
     if "component-has-children" not in env:
         env["component-has-children"] = lambda c: getattr(c, 'has_children', False)
+    # types.sx uses map-dict for record type resolution
+    if "map-dict" not in env:
+        from shared.sx.types import Lambda as _Lambda
+        def _map_dict(fn, d):
+            result = {}
+            for k, v in d.items():
+                if isinstance(fn, _Lambda):
+                    result[k] = _trampoline(_eval([fn, k, v], env))
+                else:
+                    result[k] = fn(k, v)
+            return result
+        env["map-dict"] = _map_dict
 
     # Try bootstrapped types first, fall back to eval
     try: