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/sxc/pages/docs.sx

@@ -580,6 +580,7 @@
       "foundations" (~plans/foundations/plan-foundations-content)
       "cek-reactive" (~plans/cek-reactive/plan-cek-reactive-content)
       "reactive-runtime" (~plans/reactive-runtime/plan-reactive-runtime-content)
+      "rust-wasm-host" (~plans/rust-wasm-host/plan-rust-wasm-host-content)
       :else (~plans/index/plans-index-content))))
 
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