Record draw merge helper mismatch reduction

docs/modernization/roadmap.md

@@ -3222,6 +3222,11 @@ Results:
   shared canvas blend-gate plan to decide whether they can read destination
   color through framebuffer fetch or must copy the destination texture before
   the legacy OpenGL blend draw.
+- `execute_canvas_draw_merge_branch_body()` 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. `STR-016` remains blocked until the helper
+  shape is simplified enough to move the per-layer composite block out cleanly.
 - 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