Restructure Specs section into Architecture, Core, and Adapters pages

- Add Architecture intro page explaining the spec's two-layer design
  (core language + selectable adapters) with dependency graph
- Split specs into Core (parser, eval, primitives, render) and
  Adapters (DOM, HTML, SX wire, SxEngine) overview pages
- Add individual detail pages for all adapter and engine specs
- Update nav with Architecture landing, Core, Adapters, and all
  individual spec file links

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

sx/sx/nav-data.sx

@@ -68,11 +68,17 @@
   (dict :label "The Reflexive Web" :href "/essays/reflexive-web")))
 
 (define specs-nav-items (list
+  (dict :label "Architecture" :href "/specs/")
   (dict :label "Core" :href "/specs/core")
   (dict :label "Parser" :href "/specs/parser")
   (dict :label "Evaluator" :href "/specs/evaluator")
   (dict :label "Primitives" :href "/specs/primitives")
-  (dict :label "Renderer" :href "/specs/renderer")))
+  (dict :label "Renderer" :href "/specs/renderer")
+  (dict :label "Adapters" :href "/specs/adapters")
+  (dict :label "DOM Adapter" :href "/specs/adapter-dom")
+  (dict :label "HTML Adapter" :href "/specs/adapter-html")
+  (dict :label "SX Wire Adapter" :href "/specs/adapter-sx")
+  (dict :label "SxEngine" :href "/specs/engine")))
 
 ;; Find the current nav label for a slug by matching href suffix.
 ;; Returns the label string or nil if no match.