Add Isomorphism as top-level section in sx-docs

Move isomorphic architecture roadmap and bundle analyzer from Plans
into their own top-level "Isomorphism" section. The roadmap is the
default page at /isomorphism/, bundle analyzer at /isomorphism/bundle-analyzer.

Plans section retains reader macros and SX-Activity.

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

sx/sx/nav-data.sx

@@ -102,11 +102,11 @@
   (dict :label "call/cc" :href "/specs/callcc")
   (dict :label "Deps" :href "/specs/deps")))
 
+(define isomorphism-nav-items (list
+  (dict :label "Roadmap" :href "/isomorphism/")
+  (dict :label "Bundle Analyzer" :href "/isomorphism/bundle-analyzer")))
+
 (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.")
-  (dict :label "Bundle Analyzer" :href "/plans/bundle-analyzer"
-        :summary "Live per-page component dependency analysis — see which components each page needs and the payload savings.")
   (dict :label "Reader Macros" :href "/plans/reader-macros"
         :summary "Extensible parse-time transformations via # dispatch — datum comments, raw strings, and quote shorthand.")
   (dict :label "SX-Activity" :href "/plans/sx-activity"