Narrow draw merge branch orchestration

docs/modernization/debt.md

@@ -401,6 +401,10 @@ agent or engineer to remove them without reconstructing context from chat.
   now drops the unused `draw_checkerboard` flag from the branch helper chain,
   shrinking the helper API mismatch while `Canvas::draw_merge_branch_orchestration()`
   still owns the remaining branch orchestration.
+- 2026-06-15: `DEBT-0036` was narrowed again. `Canvas::draw_merge_branch_orchestration()`
+  now drops the unused `draw_checkerboard` parameter from the internal branch
+  wrapper, shrinking the remaining call-shape mismatch while the blocked
+  draw-merge extraction still stays in `STR-016`.
 - 2026-06-14: `DEBT-0036` was narrowed again. `Canvas::draw_merge_branch_orchestration()`
   now routes the temporary erase, temporary paint, texture, and blend dispatch
   bodies through retained helpers inside `execute_canvas_draw_merge_branch_body(...)`;

docs/modernization/roadmap.md

@@ -3227,6 +3227,10 @@ Results:
   API mismatch while `Canvas::draw_merge_branch_orchestration()` still owns the
   remaining branch orchestration. `STR-016` remains blocked until the helper
   shape is simplified enough to move the per-layer composite block out cleanly.
+- `Canvas::draw_merge_branch_orchestration()` now drops the unused
+  `draw_checkerboard` parameter from the internal branch wrapper, shrinking the
+  remaining call-shape mismatch while the blocked draw-merge 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

@@ -1676,8 +1676,7 @@ void Canvas::draw_merge_branch_orchestration(
     const Brush& brush,
     const glm::mat4& ortho,
     bool use_blend,
-    bool copy_blend_destination,
+    bool copy_blend_destination)
-    bool draw_checkerboard)
 {
     if (!(m_show_tmp && m_current_layer_idx == layer_index) &&
         (!layer->m_visible ||

src/canvas.h

@@ -275,8 +275,7 @@ private:
         const Brush& brush,
         const glm::mat4& ortho,
         bool use_blend,
-        bool copy_blend_destination,
+        bool copy_blend_destination);
-        bool draw_checkerboard);
     void draw_merge_final_plane_composite(
         const glm::mat4& ortho,
         bool draw_checkerboard);