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;

docs/modernization/roadmap.md

@@ -3128,6 +3128,11 @@ Results:
   triangulation, sample-point assembly, and retained destination-copy / upload
   / draw helper handoff, while direct GL callback wiring and the remaining
   live draw ownership stay in the legacy Canvas path.
+- `Canvas::stroke_draw_mix()` now shares
+  `execute_legacy_canvas_stroke_mix_pass(...)` for visible-plane filtering,
+  retained sampler/texture-slot binding, and final plane draw ordering, while
+  mixer framebuffer/state setup and per-plane shader material/MVP preparation
+  remain in the legacy Canvas path.
 - `NodeStrokePreview::stroke_draw_mix()` now shares one local helper for mixer
   framebuffer bind/unbind, viewport/scissor/blend state, texture-slot
   binding, and final plane draw, while material planning and shader uniform

docs/modernization/tasks.md

@@ -509,6 +509,13 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `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 local to `Canvas`. Next
+  slice should target the remaining `stroke_draw_samples()` callback body or
+  any final temporary-texture composite setup without reopening landed
+  sample, sampler, dirty, face, or pad helpers.
 - 2026-06-13: `pp_paint_renderer_stroke_execution_tests` now also covers
   retained preview background capture ordering, final composite ordering, and
   preview texture-copy bind-before-copy behavior via