Isolate legacy stroke sample execution

docs/modernization/debt.md

@@ -18,6 +18,12 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-12: DEBT-0036 was narrowed again. Canvas and `NodeStrokePreview`
+  stroke sample execution now delegate optional destination copy, brush vertex
+  upload, brush-shape draw, and destination unbind through
+  `execute_legacy_canvas_stroke_sample`. Shader setup, uniforms, RTT/texture
+  ownership, dirty-box policy, and retained callback execution remain in the
+  legacy callers until a renderer-owned stroke backend replaces the adapter.
 - 2026-06-12: DEBT-0036 was narrowed again. `NodeStrokePreview` now consumes
   the same `CanvasStrokeMaterialPlan` boundary for preview dual-brush,
   each-sample pattern, and composite material decisions. Preview GL draw calls,

docs/modernization/roadmap.md

@@ -1343,8 +1343,12 @@ composite texture/uniform intent. Live `Canvas::stroke_draw`, stroke commit,
 and draw-merge paths consume that material plan for destination feedback,
 each-sample pattern, dual-brush, and final composite decisions, and
 `NodeStrokePreview` now consumes the same plan for preview material decisions.
-Actual retained OpenGL draw execution and preview texture-binding details remain
-under `DEBT-0036`.
+Canvas and `NodeStrokePreview` stroke sample execution also delegate optional
+destination copy, brush vertex upload, brush-shape draw, and destination unbind
+through `execute_legacy_canvas_stroke_sample`, creating the first retained
+OpenGL stroke execution service seam. Shader setup, uniforms, RTT/texture
+ownership, dirty-box policy, and retained callback execution remain under
+`DEBT-0036`.
 It also owns renderer API texture-format to
 OpenGL internal/pixel/component token mapping, including depth-stencil formats,
 for future backend texture objects. `Texture2D` 2D texture binding, upload,