Route NodeCanvas grid setup through helper

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. `NodeCanvas` now reuses
+  `legacy_canvas_draw_merge_services.h` for retained checkerboard grid shader
+  setup and the final cached-layer texture redraw setup. NodeCanvas still owns
+  panorama layer traversal, onion-frame drawing, blend destination copies,
+  sampler/texture binding, and draw ordering.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw_mix` now
   reuses `legacy_canvas_stroke_composite_services.h` for mixer-pass retained
   `kShader::CompDraw` binding and blend uniform writes, with the helper now

docs/modernization/roadmap.md

@@ -2996,6 +2996,10 @@ Results:
   helper for mixer-pass `CompDraw` setup, while preserving its caller-specific
   texture slot uniforms. Mixer framebuffer/scissor state, sampler and texture
   binding, and draw execution remain retained.
+- `NodeCanvas` panorama drawing now uses the same retained draw-merge shader
+  setup helper for checkerboard grids and final cached-layer texture redraws;
+  onion-frame traversal, blend copies, texture binding, and draw order remain
+  retained in the node.
 - 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.