Add Reactive Islands plan to sx-docs

Design plan for client-side state via signals and islands — a second
sliding bar (reactivity) orthogonal to the existing isomorphism bar.
Covers signal primitives, defisland, shared state, reactive DOM
rendering, SSR hydration, and spec architecture.

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

sx/sxc/pages/docs.sx

@@ -581,6 +581,7 @@
     "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)))
 
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