Move reactive runtime from plans to applications section

Promotes reactive-runtime from a plan page to a full applications section
with 7 nav items (ref, foreign FFI, state machines, commands, render loop,
keyed lists, app shell) and its own page function.

Co-Authored-By: Claude Opus 4.6 (1M context) <noreply@anthropic.com>

sx/sx/page-functions.sx

@@ -468,6 +468,11 @@
       '(~sx-pub/overview-content)
       nil)))
 
+;; Reactive Runtime (under applications)
+;; Convention: ~reactive-runtime/{slug}-content
+(define reactive-runtime
+  (make-page-fn "~reactive-runtime/overview-content" "~reactive-runtime/" nil "-content"))
+
 ;; Essays (under etc)
 ;; Convention: ~essays/{slug}/essay-{slug}
 (define essay