Add CSSX as top-level docs section with patterns, async, and live examples

New top-level section in sx-docs: /cssx/ with 6 pages:
- Overview: the idea, what changed, advantages
- Patterns: class mapping, data-driven, style functions, responsive, emitting CSS
- Async CSS: components that fetch/cache CSS before rendering via ~suspense
- Live Styles: SSE/WS examples for real-time style updates (noted as future)
- Comparisons: vs styled-components, CSS Modules, Tailwind, Vanilla Extract
- Philosophy: proof by deletion, the right abstraction level

Also:
- Remove CSSX from specs nav (spec file deleted)
- Fix renderer spec prose (no longer mentions StyleValue)
- Update On-Demand CSS essay summary
- Mark CSSX Components plan as done

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

sx/sxc/pages/docs.sx

@@ -286,6 +286,39 @@
     "no-alternative" (~essay-no-alternative)
     :else (~essays-index-content)))
 
+;; ---------------------------------------------------------------------------
+;; CSSX section
+;; ---------------------------------------------------------------------------
+
+(defpage cssx-index
+  :path "/cssx/"
+  :auth :public
+  :layout (:sx-section
+    :section "CSSX"
+    :sub-label "CSSX"
+    :sub-href "/cssx/"
+    :sub-nav (~section-nav :items cssx-nav-items :current "Overview")
+    :selected "Overview")
+  :content (~cssx-overview-content))
+
+(defpage cssx-page
+  :path "/cssx/<slug>"
+  :auth :public
+  :layout (:sx-section
+    :section "CSSX"
+    :sub-label "CSSX"
+    :sub-href "/cssx/"
+    :sub-nav (~section-nav :items cssx-nav-items
+               :current (find-current cssx-nav-items slug))
+    :selected (or (find-current cssx-nav-items slug) ""))
+  :content (case slug
+    "patterns" (~cssx-patterns-content)
+    "async" (~cssx-async-content)
+    "live" (~cssx-live-content)
+    "comparisons" (~cssx-comparison-content)
+    "philosophy" (~cssx-philosophy-content)
+    :else (~cssx-overview-content)))
+
 ;; ---------------------------------------------------------------------------
 ;; Specs section
 ;; ---------------------------------------------------------------------------