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_py.py

@@ -881,6 +881,7 @@ PRIMITIVES["zero?"] = lambda n: n == 0
     "core.strings": '''
 # core.strings
 PRIMITIVES["str"] = sx_str
+PRIMITIVES["char-from-code"] = lambda n: chr(_b_int(n))
 PRIMITIVES["upper"] = lambda s: str(s).upper()
 PRIMITIVES["lower"] = lambda s: str(s).lower()
 PRIMITIVES["trim"] = lambda s: str(s).strip()