Replace invoke with cek-call in reactive island primitives

All signal operations (computed, effect, batch, etc.) now dispatch
function calls through cek-call, which routes SX lambdas via cek-run
and native callables via apply. This replaces the invoke shim.

Key changes:
- cek.sx: add cek-call (defined before reactive-shift-deref), replace
  invoke in subscriber disposal and ReactiveResetFrame handler
- signals.sx: replace all 11 invoke calls with cek-call
- js.sx: fix octal escape in js-quote-string (char-from-code 0)
- platform_js.py: fix JS append to match Python (list concat semantics),
  add Continuation type guard in PLATFORM_CEK_JS, add scheduleIdle
  safety check, module ordering (cek before signals)
- platform_py.py: fix ident-char regex (remove [ ] from valid chars),
  module ordering (cek before signals)
- run_js_sx.py: emit PLATFORM_CEK_JS before transpiled spec files
- page-functions.sx: add cek and provide page functions for SX URLs

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

shared/sx/ref/parser.sx

@@ -25,6 +25,7 @@
 ;;   comment    → ';' to end of line (discarded)
 ;;
 ;; Quote sugar:
+;;   'expr      → (quote expr)
 ;;   `expr      → (quasiquote expr)
 ;;   ,expr      → (unquote expr)
 ;;   ,@expr     → (splice-unquote expr)
@@ -267,6 +268,11 @@
                 (= ch ":")
                   (read-keyword)
 
+                ;; Quote sugar
+                (= ch "'")
+                  (do (set! pos (inc pos))
+                      (list (make-symbol "quote") (read-expr)))
+
                 ;; Quasiquote sugar
                 (= ch "`")
                   (do (set! pos (inc pos))
@@ -395,7 +401,7 @@
 ;;     True for: a-z A-Z _ ~ * + - > < = / ! ? &
 ;;
 ;;   (ident-char? ch) → boolean
-;;     True for: ident-start chars plus: 0-9 . : / [ ] # ,
+;;     True for: ident-start chars plus: 0-9 . : / # ,
 ;;
 ;; Constructors (provided by the SX runtime):
 ;;   (make-symbol name)  → Symbol value