Route texture alpha setup through helper

docs/modernization/roadmap.md

@@ -3051,6 +3051,11 @@ Results:
   share the retained draw-merge texture shader setup helper for `Texture`,
   `Tex`, and `MVP` setup. Texture/sampler binding, viewport/state setup, and
   draw execution remain in the node paths.
+- Retained `TextureAlpha` setup now shares the draw-merge shader setup helper
+  for Canvas draw-merge layer redraws, depth export merged-layer rendering,
+  NodeCanvas cached/live layer redraws, and desktop VR layer redraws, while
+  sampler/texture binding, render-task ordering, per-frame alpha updates, and
+  draw execution remain in retained code.
 - `NodeCanvas` density-resolve drawing and desktop VR UI drawing now share the
   retained draw-merge texture shader setup helper, while render target,
   sampler/texture binding, viewport/state restoration, and draw execution