Add Reactive Islands as top-level section in sx-docs

- New nav entry in ~sx-main-nav (layouts.sx)
- Nav items: Overview, Demo, Plan link (nav-data.sx)
- Overview page: architecture quadrant, four levels, signal primitives,
  island lifecycle, implementation status table with done/todo
- Demo page: annotated code examples for signal+computed+effect, batch,
  cleanup, computed chains, defisland, test suite
- defpage routes: /reactive-islands/ and /reactive-islands/<slug>

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

sx/sx/nav-data.sx

@@ -164,6 +164,14 @@
   (dict :label "Reactive Islands" :href "/plans/reactive-islands"
         :summary "Client-side state via signals and islands — a sliding bar between hypermedia and React, orthogonal to the server/client rendering bar.")))
 
+(define reactive-islands-nav-items (list
+  (dict :label "Overview" :href "/reactive-islands/"
+        :summary "Architecture, four levels (L0-L3), and current implementation status.")
+  (dict :label "Demo" :href "/reactive-islands/demo"
+        :summary "Live demonstration of signals, computed, effects, batch, and defisland — all transpiled from spec.")
+  (dict :label "Plan" :href "/plans/reactive-islands"
+        :summary "The full design document — rendering boundary, state flow, signal primitives, island lifecycle.")))
+
 (define bootstrappers-nav-items (list
   (dict :label "Overview" :href "/bootstrappers/")
   (dict :label "JavaScript" :href "/bootstrappers/javascript")