Isolate draw merge background shader setup

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge` checkerboard
+  background shader setup and final merged-texture redraw setup now route
+  through `legacy_canvas_draw_merge_services.h`. The retained Canvas path still
+  owns draw-merge layer iteration, blend-gate branching, framebuffer copies,
+  sampler/texture binding, and draw ordering.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::layer_merge` now reuses
   `legacy_canvas_stroke_composite_services.h` for retained layer-merge
   `kShader::CompDraw` binding and source/destination blend uniform writes.

docs/modernization/roadmap.md

@@ -2988,6 +2988,10 @@ Results:
   for its source-over-destination `CompDraw` uniform setup, while keeping the
   existing layer dirty mutation, framebuffer copy, texture binding, and draw
   order local to the Canvas layer-merge path.
+- `Canvas::draw_merge` now shares a retained draw-merge shader setup helper for
+  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 thumbnail layer blending now uses the same canvas destination-feedback
   plan for framebuffer-fetch versus texture-copy decisions; the thumbnail draw
   itself still executes through retained OpenGL canvas code under DEBT-0036.