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>

sx/sx/page-functions.sx

@@ -60,6 +60,18 @@
         "phase2" '(~reactive-islands/phase2/reactive-islands-phase2-content)
         :else '(~reactive-islands/index/reactive-islands-index-content)))))
 
+(define cek
+  (fn (slug)
+    (if (nil? slug)
+      '(~geography/cek/cek-content)
+      (case slug
+        "demo" '(~geography/cek/cek-demo-content)
+        :else '(~geography/cek/cek-content)))))
+
+(define provide
+  (fn (content)
+    (if (nil? content) '(~geography/provide-content) content)))
+
 (define scopes
   (fn (content)
     (if (nil? content) '(~geography/scopes-content) content)))