Add style matrix validation surface

docs/rfcs/multi-style-architecture.md

@@ -511,7 +511,7 @@ Deliverables:
 
 Status:
 
-- not started
+- completed
 
 ### Phase 4: Expand Coverage
 
@@ -605,7 +605,9 @@ As of 2026-03-20, the project is at this point:
 - docs switching surface added in `Foundation/Style Contract`
 - pilot recipe extraction completed for `Button`, `Card`, `Input`, `Dialog`, `Switch`,
   and `Skeleton`
+- screenshot-friendly validation surface added in `Foundation/Style Matrix`
+- scoped `data-motion="reduced"` now works for nested docs wrappers
 - broader component-library rollout still pending
 
-The next implementation task should be Phase 3: add a dedicated comparison-oriented docs
-validation surface for skins and motion modes.
+The next implementation task should be Phase 4: expand the skin-aware pattern across the
+broader component library.