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/sxc/pages/docs.sx

@@ -608,6 +608,35 @@
     "reactive-islands" (~plan-reactive-islands-content)
     :else (~plans-index-content)))
 
+;; ---------------------------------------------------------------------------
+;; Reactive Islands section
+;; ---------------------------------------------------------------------------
+
+(defpage reactive-islands-index
+  :path "/reactive-islands/"
+  :auth :public
+  :layout (:sx-section
+    :section "Reactive Islands"
+    :sub-label "Reactive Islands"
+    :sub-href "/reactive-islands/"
+    :sub-nav (~section-nav :items reactive-islands-nav-items :current "Overview")
+    :selected "Overview")
+  :content (~reactive-islands-index-content))
+
+(defpage reactive-islands-page
+  :path "/reactive-islands/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Reactive Islands"
+    :sub-label "Reactive Islands"
+    :sub-href "/reactive-islands/"
+    :sub-nav (~section-nav :items reactive-islands-nav-items
+               :current (find-current reactive-islands-nav-items slug))
+    :selected (or (find-current reactive-islands-nav-items slug) ""))
+  :content (case slug
+    "demo" (~reactive-islands-demo-content)
+    :else (~reactive-islands-index-content)))
+
 ;; ---------------------------------------------------------------------------
 ;; Testing section
 ;; ---------------------------------------------------------------------------