Route canvas blend gate through paint renderer

docs/modernization/roadmap.md

@@ -873,8 +873,11 @@ stroke composite plan that decides whether a stroke/layer blend can use
 fixed-function blending or needs framebuffer-fetch/ping-pong destination
 feedback. `pano_cli plan-stroke-composite` exposes the same decision for
 automation, including layer blend, stroke blend, dual-brush, and pattern-blend
-inputs. Live canvas stroke execution is still legacy OpenGL and remains tracked
-by DEBT-0036 until the app calls through this paint-renderer boundary.
+inputs. Live `Canvas::draw_merge` now uses this planner for its existing
+shader-blend gate for layer and primary-brush blend modes while preserving the
+legacy trigger policy; actual canvas stroke execution, dual-brush feedback, and
+pattern feedback are still legacy OpenGL and remain tracked by DEBT-0036 until
+the app calls through renderer services for the whole compositing path.
 The existing renderer classes are not yet fully
 behind the renderer interfaces.
 
@@ -1595,6 +1598,10 @@ Results:
 - Canvas layer merge rendering and explicit layer-merge compositing now route
   depth/blend state, active texture units, fallback 2D texture unbinds, and
   merge framebuffer copy targets through the renderer GL backend mapping.
+- Canvas draw-merge shader-blend selection now consumes the extracted
+  `pp_paint_renderer` stroke composite planner for current layer and primary
+  brush blend modes, while preserving legacy OpenGL compositing execution under
+  DEBT-0036.
 - Canvas equirectangular import drawing and depth export rendering now route
   depth/blend state and active texture units through the renderer GL backend
   mapping.