Spec event bridge and named stores, move plan to reactive islands section

- signals.sx: add def-store/use-store/clear-stores (L3 named stores)
  and emit-event/on-event/bridge-event (lake→island DOM events)
- reactive-islands.sx: add event bridge, named stores, and plan pages
- Remove ~plan-reactive-islands-content from plans.sx
- Update nav-data.sx and docs.sx routing accordingly

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

sx/sxc/pages/docs.sx

@@ -605,7 +605,6 @@
     "social-sharing" (~plan-social-sharing-content)
     "sx-ci" (~plan-sx-ci-content)
     "live-streaming" (~plan-live-streaming-content)
-    "reactive-islands" (~plan-reactive-islands-content)
     :else (~plans-index-content)))
 
 ;; ---------------------------------------------------------------------------
@@ -635,6 +634,9 @@
     :selected (or (find-current reactive-islands-nav-items slug) ""))
   :content (case slug
     "demo" (~reactive-islands-demo-content)
+    "event-bridge" (~reactive-islands-event-bridge-content)
+    "named-stores" (~reactive-islands-named-stores-content)
+    "plan" (~reactive-islands-plan-content)
     :else (~reactive-islands-index-content)))
 
 ;; ---------------------------------------------------------------------------