Use blend gate plan for canvas copy decisions

docs/modernization/roadmap.md

@@ -882,7 +882,8 @@ the app calls through renderer services for the whole compositing path.
 mapping from persisted layer and brush blend indices to that planner, including
 fallback behavior for unknown nonzero indices. Both `Canvas::draw_merge` and
 `NodeCanvas` panorama rendering consume that shared gate, so the live app no
-longer has duplicate local blend-trigger logic.
+longer has duplicate local blend-trigger logic or duplicate destination-copy
+versus framebuffer-fetch decisions in those paths.
 The OpenGL shader initialization path now stores a renderer-neutral
 `RenderDeviceFeatures` snapshot converted by `pp_renderer_gl`, and those live
 canvas gates consume that snapshot instead of rebuilding feature flags from
@@ -1618,6 +1619,10 @@ Results:
   feature snapshot through the legacy shader manager, and live canvas blend
   gates consume that `RenderDeviceFeatures` value instead of hand-built
   framebuffer-fetch/texture-copy flags.
+- Canvas draw-merge and `NodeCanvas` panorama shader-blend paths now use the
+  shared canvas blend-gate plan to decide whether they can read destination
+  color through framebuffer fetch or must copy the destination texture before
+  the legacy OpenGL blend draw.
 - Canvas equirectangular import drawing and depth export rendering now route
   depth/blend state and active texture units through the renderer GL backend
   mapping.