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/parser.py

@@ -106,6 +106,14 @@ def _unescape_string(s: str) -> str:
     while i < len(s):
         if s[i] == "\\" and i + 1 < len(s):
             nxt = s[i + 1]
+            if nxt == "u" and i + 5 < len(s):
+                hex_str = s[i + 2:i + 6]
+                try:
+                    out.append(chr(int(hex_str, 16)))
+                    i += 6
+                    continue
+                except ValueError:
+                    pass  # fall through to default handling
             out.append(_ESCAPE_MAP.get(nxt, nxt))
             i += 2
         else: