Route layer merge composite 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::layer_merge` now reuses
+  `legacy_canvas_stroke_composite_services.h` for retained layer-merge
+  `kShader::CompDraw` binding and source/destination blend uniform writes.
+  Render-task ordering, dirty face/box mutation, framebuffer copies, sampler
+  binding, texture binding, and draw execution remain in the retained Canvas
+  layer-merge path.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_commit` now routes
   its retained per-face commit order through
   `execute_legacy_canvas_stroke_commit_sequence`, consuming the tested

docs/modernization/roadmap.md

@@ -2984,6 +2984,10 @@ Results:
   through retained callbacks, so the legacy body no longer owns the loop order
   directly. The callbacks still execute the existing OpenGL RTT, texture,
   sampler, shader, history, and layer mutation work under DEBT-0036.
+- `Canvas::layer_merge` now shares the retained stroke composite shader helper
+  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 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.