Share retained stroke sample and mix helpers

docs/modernization/debt.md

@@ -23,6 +23,16 @@ agent or engineer to remove them without reconstructing context from chat.
   retained stroke execution helpers; concrete GL object mapping, framebuffer
   ownership, shader timing, and final draw execution remain retained in
   `Canvas`.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::stroke_draw_samples()`
+  now routes polygon triangulation, sample-point assembly, and retained
+  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. `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;
+  material planning and shader uniform setup remain retained in the preview
+  node.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::stroke_draw_samples`
   now routes destination bind/unbind, framebuffer copy callback wrapping,
   sample-point assembly, and brush-vertex upload/draw through one local helper;

docs/modernization/roadmap.md

@@ -3123,6 +3123,15 @@ Results:
   texture-input dispatch now shares retained stroke execution helpers, while
   concrete GL object mapping, framebuffer ownership, shader timing, and final
   draw execution remain in the legacy Canvas path.
+- `Canvas::stroke_draw_samples()` now shares
+  `execute_legacy_canvas_stroke_sample_polygon(...)` for polygon
+  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.
+- `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
+  setup remain in the preview node.
 - `pp_paint_renderer_stroke_execution_tests` now covers retained stroke texture
   input binding order, sample execution destination-copy behavior, live-pass
   face-framebuffer dirty tracking, and pad-face destination-copy behavior

docs/modernization/tasks.md

@@ -509,6 +509,20 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::stroke_draw_samples()` now routes polygon
+  triangulation, sample-point assembly, and the retained destination-copy /
+  upload / draw helper handoff through
+  `execute_legacy_canvas_stroke_sample_polygon(...)`; direct GL callback
+  wiring and the remaining live draw ownership stay local to `Canvas`. Next
+  slice should target the remaining callback body or final temporary-texture
+  composite setup without reopening the landed sampler, dirty, face, or pad
+  helpers.
+- 2026-06-13: `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; material planning and shader
+  uniform setup remain local to the preview node. Next slice should target the
+  remaining mix-pass material/setup orchestration without reopening the landed
+  preview live-pass, binding, sample, or final composite helpers.
 - 2026-06-13: `pp_paint_renderer_stroke_execution_tests` now also covers
   retained texture-dispatch activation order and sampler-dispatch routing
   across brush tip, destination, pattern, and mixer helper inputs. Next test