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/layouts.sx

@@ -12,7 +12,7 @@
            (dict :label "Protocols" :href "/protocols/wire-format")
            (dict :label "Examples" :href "/examples/click-to-load")
            (dict :label "Essays" :href "/essays/sx-sucks")
-           (dict :label "Specs" :href "/specs/core"))))
+           (dict :label "Specs" :href "/specs/"))))
     (<> (map (lambda (item)
       (~nav-link
         :href (get item "href")