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/run_js_sx.py

@@ -190,6 +190,10 @@ def compile_ref_to_js(
     if has_parser:
         parts.append(adapter_platform["parser"])
 
+    # CEK platform aliases must come before transpiled cek.sx (which uses them)
+    if has_cek:
+        parts.append(PLATFORM_CEK_JS)
+
     # Translate each spec file using js.sx
     for filename, label in sx_files:
         filepath = os.path.join(ref_dir, filename)
@@ -216,9 +220,6 @@ def compile_ref_to_js(
         if name in adapter_set and name in adapter_platform:
             parts.append(adapter_platform[name])
 
-    if has_cek:
-        parts.append(PLATFORM_CEK_JS)
-
     parts.append(fixups_js(has_html, has_sx, has_dom, has_signals, has_deps, has_page_helpers))
     if has_cek:
         parts.append(CEK_FIXUPS_JS)