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/layouts.sx

@@ -14,6 +14,7 @@
            (dict :label "Essays" :href "/essays/")
            (dict :label "Specs" :href "/specs/")
            (dict :label "Bootstrappers" :href "/bootstrappers/")
+           (dict :label "Isomorphism" :href "/isomorphism/")
            (dict :label "Plans" :href "/plans/"))))
     (<> (map (lambda (item)
       (~nav-link