Add CSSX Components plan: styling via defcomp instead of opaque style dict

Replace the existing CSSX plan with a component-based approach where styling
is handled by regular defcomp components that apply classes, respond to data,
and compose naturally — eliminating opaque hash-based class names.

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

sx/sxc/pages/docs.sx

@@ -531,6 +531,7 @@
     "fragment-protocol" (~plan-fragment-protocol-content)
     "glue-decoupling" (~plan-glue-decoupling-content)
     "social-sharing" (~plan-social-sharing-content)
+    "cssx-components" (~plan-cssx-components-content)
     :else (~plans-index-content)))
 
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