Remove dead draw merge temporary paint parameter

docs/modernization/debt.md

@@ -406,8 +406,8 @@ agent or engineer to remove them without reconstructing context from chat.
   wrapper, shrinking the remaining call-shape mismatch while the blocked
   draw-merge extraction still stays in `STR-016`.
 - 2026-06-15: `DEBT-0036` was narrowed again. `Canvas::draw_merge_temporary_paint_branch()`
-  now drops the unused `draw_checkerboard` parameter from the temporary-paint
-  wrapper, shrinking the remaining draw-merge helper mismatch while the
+  now drops the unused `copy_blend_destination` parameter from the temporary-paint
+  wrapper, shrinking the remaining draw-merge helper API mismatch while the
   blocked extraction still stays in `STR-016`.
 - 2026-06-15: `DEBT-0036` was narrowed again. The stale
   `make_canvas_draw_merge_branch_dispatch(...)` forward declaration in

docs/modernization/roadmap.md

@@ -3235,9 +3235,9 @@ Results:
   remaining call-shape mismatch while the blocked draw-merge extraction still
   stays in `STR-016`.
 - `Canvas::draw_merge_temporary_paint_branch()` now drops the unused
-  `draw_checkerboard` parameter from the temporary-paint wrapper, shrinking the
-  remaining draw-merge helper mismatch while the blocked extraction still stays
-  in `STR-016`.
+  `copy_blend_destination` parameter from the temporary-paint wrapper,
+  shrinking the remaining draw-merge helper API mismatch while the blocked
+  extraction still stays in `STR-016`.
 - Canvas main-brush, dual-brush, and stroke-pad draw paths now use the tested
   `pp_paint_renderer` stroke-feedback plan to decide whether framebuffer fetch
   supplies destination color or the legacy OpenGL path must copy the target

src/canvas.cpp

@@ -1653,8 +1653,7 @@ void Canvas::draw_merge_temporary_paint_branch(
     int plane_index,
     std::shared_ptr<Layer> layer,
     const Brush& brush,
-    const glm::mat4& ortho,
-    bool copy_blend_destination)
+    const glm::mat4& ortho)
 {
     pp::panopainter::execute_legacy_canvas_draw_merge_temporary_composite(
         make_canvas_draw_merge_temporary_paint_request(

src/canvas.h

@@ -265,8 +265,7 @@ private:
         int plane_index,
         std::shared_ptr<Layer> layer,
         const Brush& brush,
-        const glm::mat4& ortho,
-        bool copy_blend_destination);
+        const glm::mat4& ortho);
     void draw_merge_branch_orchestration(
         int plane_index,
         int layer_index,