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/sxc/pages/docs.sx

@@ -386,3 +386,32 @@
       (~bootstrapper-js-content
         :bootstrapper-source bootstrapper-source
         :bootstrapped-output bootstrapped-output))))
+
+;; ---------------------------------------------------------------------------
+;; Plans section
+;; ---------------------------------------------------------------------------
+
+(defpage plans-index
+  :path "/plans/"
+  :auth :public
+  :layout (:sx-section
+    :section "Plans"
+    :sub-label "Plans"
+    :sub-href "/plans/"
+    :sub-nav (~section-nav :items plans-nav-items :current "")
+    :selected "")
+  :content (~plans-index-content))
+
+(defpage plan-page
+  :path "/plans/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "Plans"
+    :sub-label "Plans"
+    :sub-href "/plans/"
+    :sub-nav (~section-nav :items plans-nav-items
+               :current (find-current plans-nav-items slug))
+    :selected (or (find-current plans-nav-items slug) ""))
+  :content (case slug
+    "isomorphic-architecture" (~plan-isomorphic-content)
+    :else (~plans-index-content)))