Share retained stroke mix pass helper

docs/modernization/debt.md

@@ -28,6 +28,11 @@ agent or engineer to remove them without reconstructing context from chat.
   destination-copy / upload / draw helper handoff through
   `execute_legacy_canvas_stroke_sample_polygon(...)`; direct GL callback
   wiring and remaining live draw ownership remain retained in `Canvas`.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw_mix()` now
+  routes visible-plane filtering, retained sampler/texture-slot binding, and
+  final plane draw ordering through `execute_legacy_canvas_stroke_mix_pass(...)`;
+  mixer framebuffer/state setup and per-plane shader material/MVP preparation
+  remain retained in `Canvas`.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::stroke_draw_mix()`
   now routes mixer framebuffer bind/unbind, viewport/scissor/blend state,
   texture-slot binding, and final plane draw through one local helper;