Collapse draw merge layer composite shell

docs/modernization/debt.md

@@ -89,6 +89,10 @@ agent or engineer to remove them without reconstructing context from chat.
   the remaining temporary erase and paint callback bundle through
   `execute_legacy_canvas_draw_merge_temporary_composite(...)`; branch
   selection remains in `Canvas`.
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
+  the layer-composite shell through a local wrapper around
+  `execute_legacy_canvas_draw_merge_layer_composite(...)`; the final branch
+  selection remains in `Canvas`.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::draw_stroke_immediate()`
   now routes retained preview feedback/material/composite planning plus stroke
   shader uniform assembly through

docs/modernization/tasks.md

@@ -670,6 +670,11 @@ Progress Notes:
   `execute_legacy_canvas_draw_merge_temporary_composite(...)`; only the
   branch selection remains inline. Next slice should target another narrow
   draw-merge seam without reopening the landed temporary-composite helper.
+- 2026-06-13: `Canvas::draw_merge()` now routes the layer-composite shell
+  through a local wrapper around
+  `execute_legacy_canvas_draw_merge_layer_composite(...)`; only the final
+  branch selection remains inline. Next slice should target another narrow
+  draw-merge seam without reopening the landed temporary-composite helper.
 - 2026-06-13: `pp_paint_renderer_stroke_execution_tests` now also covers
   retained frame-plan assembly for previous-sample projection mode and zoom
   scaling. Next slice should target the remaining preview/Canvas stroke

src/canvas.cpp

@@ -555,6 +555,19 @@ std::array<std::vector<vertex_t>, 6> Canvas::stroke_draw_project(std::array<vert
     return ret;
 }
 
+static void execute_canvas_draw_merge_layer_composite(
+    bool is_temporary_erase,
+    bool is_temporary_paint,
+    bool use_blend,
+    const pp::panopainter::LegacyCanvasDrawMergeLayerCompositeExecution& execution)
+{
+    pp::panopainter::execute_legacy_canvas_draw_merge_layer_composite(
+        is_temporary_erase,
+        is_temporary_paint,
+        use_blend,
+        execution);
+}
+
 glm::vec4 Canvas::stroke_draw_samples(
     int i,
     std::vector<vertex_t>& P,
@@ -1429,7 +1442,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                 m_merge_rtt.clear();
             }
 
-                pp::panopainter::execute_legacy_canvas_draw_merge_layer_composite(
+                execute_canvas_draw_merge_layer_composite(
             m_current_stroke && m_current_mode == kCanvasMode::Erase && m_show_tmp && m_current_layer_idx == layer_index,
             m_current_stroke && m_show_tmp && m_current_layer_idx == layer_index,
             use_blend,