Route draw merge blend setup through helper

docs/modernization/debt.md

@@ -18,6 +18,12 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge` non-stroke
+  `TextureBlend` shader setup now routes through
+  `legacy_canvas_draw_merge_services.h`, preserving the optional `TexBG`
+  uniform only when copy-based blend destination feedback is active. Canvas
+  still owns draw-merge layer iteration, framebuffer copies, sampler/texture
+  binding, and draw ordering.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeCanvas` non-stroke
   `TextureBlend` shader setup now routes through
   `legacy_canvas_draw_merge_services.h`, including the optional destination

docs/modernization/roadmap.md

@@ -2992,6 +2992,11 @@ Results:
   checkerboard backgrounds and the final merged texture redraw over the grid;
   layer traversal, blend destination copies, sampler/texture binding, and draw
   ordering remain in the legacy Canvas path.
+- `Canvas::draw_merge` non-stroke layer blending now shares the retained
+  draw-merge shader setup helper for `TextureBlend` uniforms, preserving the
+  optional `TexBG` uniform when copy-based blend destination feedback is active.
+  Layer traversal, framebuffer copies, sampler/texture binding, and draw
+  ordering remain in the legacy Canvas path.
 - `Canvas::stroke_draw_mix` now shares the retained stroke composite shader
   helper for mixer-pass `CompDraw` setup, while preserving its caller-specific
   texture slot uniforms. Mixer framebuffer/scissor state, sampler and texture