Extract preview mix pass execution

docs/modernization/debt.md

@@ -201,6 +201,10 @@ agent or engineer to remove them without reconstructing context from chat.
   `execute_legacy_canvas_draw_merge_layer_texture(...)`; per-plane iteration,
   temporary-stroke branch selection, and concrete layer RTT ownership remain in
   the legacy Canvas path.
+- 2026-06-14: DEBT-0036 was narrowed again. `NodeStrokePreview::stroke_draw_mix()`
+  now routes the mix-pass execution request through
+  `make_stroke_draw_mix_execution_request(...)`; the retained path still owns
+  the concrete framebuffer, viewport, and draw ordering.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` temporary
   composite setup now routes through `execute_canvas_draw_merge_temporary_composite(...)`;
   setup, sampler, texture, draw, and unbind callbacks still remain retained in