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/sxc/pages/docs.sx

@@ -253,11 +253,10 @@
   :layout (:sx-section
     :section "Specs"
     :sub-label "Specs"
-    :sub-href "/specs/core"
-    :sub-nav (~section-nav :items specs-nav-items :current "Core")
-    :selected "Core")
-  :data (spec-data "core")
-  :content (~spec-core-content :spec-files spec-files))
+    :sub-href "/specs/"
+    :sub-nav (~section-nav :items specs-nav-items :current "Architecture")
+    :selected "Architecture")
+  :content (~spec-architecture-content))
 
 (defpage specs-page
   :path "/specs/<slug>"
@@ -265,7 +264,7 @@
   :layout (:sx-section
     :section "Specs"
     :sub-label "Specs"
-    :sub-href "/specs/core"
+    :sub-href "/specs/"
     :sub-nav (~section-nav :items specs-nav-items
                :current (find-current specs-nav-items slug))
     :selected (or (find-current specs-nav-items slug) ""))
@@ -273,7 +272,8 @@
   :content (if spec-not-found
     (~spec-not-found :slug slug)
     (case slug
-      "core" (~spec-core-content :spec-files spec-files)
+      "core" (~spec-overview-content :spec-files spec-files)
+      "adapters" (~spec-overview-content :spec-files spec-files)
       :else (~spec-detail-content
         :spec-title spec-title
         :spec-desc spec-desc