Add Plans section to SX docs with isomorphic architecture roadmap

New top-level nav section at /plans/ with the 6-phase isomorphic
architecture plan: component distribution, smart boundary, SPA routing,
client IO bridge, streaming suspense, and full isomorphism.

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

sx/sx/nav-data.sx

@@ -101,6 +101,10 @@
   (dict :label "Continuations" :href "/specs/continuations")
   (dict :label "call/cc" :href "/specs/callcc")))
 
+(define plans-nav-items (list
+  (dict :label "Isomorphic Architecture" :href "/plans/isomorphic-architecture"
+        :summary "Making the server/client boundary a sliding window — per-page bundles, smart expansion, SPA routing, client IO, streaming suspense.")))
+
 (define bootstrappers-nav-items (list
   (dict :label "Overview" :href "/bootstrappers/")
   (dict :label "JavaScript" :href "/bootstrappers/javascript")