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/layouts.sx

@@ -13,7 +13,8 @@
            (dict :label "Examples" :href "/examples/click-to-load")
            (dict :label "Essays" :href "/essays/")
            (dict :label "Specs" :href "/specs/")
-           (dict :label "Bootstrappers" :href "/bootstrappers/"))))
+           (dict :label "Bootstrappers" :href "/bootstrappers/")
+           (dict :label "Plans" :href "/plans/"))))
     (<> (map (lambda (item)
       (~nav-link
         :href (get item "href")