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/platform_py.py

@@ -1038,7 +1038,7 @@ PLATFORM_PARSER_PY = '''
 import re as _re_parser
 
 _IDENT_START_RE = _re_parser.compile(r"[a-zA-Z_~*+\\-><=/!?&]")
-_IDENT_CHAR_RE = _re_parser.compile(r"[a-zA-Z0-9_~*+\\-><=/!?.:&/\\[\\]#,]")
+_IDENT_CHAR_RE = _re_parser.compile(r"[a-zA-Z0-9_~*+\\-><=/!?.:&/#,]")
 
 
 def ident_start_p(ch):
@@ -1626,7 +1626,7 @@ SPEC_MODULES = {
 # Explicit ordering for spec modules with dependencies.
 # Modules listed here are emitted in this order; any not listed use alphabetical.
 SPEC_MODULE_ORDER = [
-    "deps", "engine", "frames", "page-helpers", "router", "signals", "types", "cek",
+    "deps", "engine", "frames", "page-helpers", "router", "cek", "signals", "types",
 ]
 
 EXTENSION_NAMES = {"continuations"}