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/plans.sx

@@ -625,7 +625,7 @@
         (div :class "flex items-center gap-2 mb-2"
           (span :class "inline-block px-2 py-0.5 rounded text-xs font-bold bg-green-600 text-white uppercase" "Complete")
           (a :href "/specs/deps" :class "text-green-700 underline text-sm font-medium" "View canonical spec: deps.sx")
-          (a :href "/plans/bundle-analyzer" :class "text-green-700 underline text-sm font-medium" "Live bundle analyzer"))
+          (a :href "/isomorphism/bundle-analyzer" :class "text-green-700 underline text-sm font-medium" "Live bundle analyzer"))
         (p :class "text-green-900 font-medium" "What it enables")
         (p :class "text-green-800" "Per-page component bundles instead of sending every definition to every page. Smaller payloads, faster boot, better cache hit rates."))
 
@@ -677,7 +677,7 @@
           (li "15 dedicated tests: scan, transitive closure, circular deps, compute-all, components-needed")
           (li "Bootstrapped output verified on both host targets")
           (li "Full test suite passes with zero regressions")
-          (li (a :href "/plans/bundle-analyzer" :class "text-violet-700 underline" "Live bundle analyzer") " shows real per-page savings on this app"))))
+          (li (a :href "/isomorphism/bundle-analyzer" :class "text-violet-700 underline" "Live bundle analyzer") " shows real per-page savings on this app"))))
 
     ;; -----------------------------------------------------------------------
     ;; Phase 2