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/nav-data.sx

@@ -236,7 +236,9 @@
   (dict :label "Deref as Shift" :href "/sx/(etc.(plan.cek-reactive))"
         :summary "Phase B: replace explicit effect wrapping with implicit continuation capture. Deref inside reactive-reset performs shift, capturing the rest of the expression as the subscriber.")
   (dict :label "Reactive Runtime" :href "/sx/(etc.(plan.reactive-runtime))"
-        :summary "Seven feature layers — ref, foreign FFI, state machines, commands with undo/redo, render loops, keyed lists, client-first app shell. Zero new platform primitives.")))
+        :summary "Seven feature layers — ref, foreign FFI, state machines, commands with undo/redo, render loops, keyed lists, client-first app shell. Zero new platform primitives.")
+  (dict :label "Rust/WASM Host" :href "/sx/(etc.(plan.rust-wasm-host))"
+        :summary "Bootstrap the SX spec to Rust, compile to WASM, replace sx-browser.js. Shared platform layer for DOM, phased rollout from parse to full parity.")))
 
 (define reactive-islands-nav-items (list
   (dict :label "Overview" :href "/sx/(geography.(reactive))"