Wrap remaining draw merge callbacks

docs/modernization/debt.md

@@ -93,6 +93,10 @@ agent or engineer to remove them without reconstructing context from chat.
   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. `Canvas::draw_merge()` now routes
+  the layer texture, layer blend, and final-plane composite callbacks through
+  local wrappers around the retained helpers; only branch selection glue
+  remains in the legacy Canvas path.
 - 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

@@ -675,6 +675,10 @@ Progress Notes:
   `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: `Canvas::draw_merge()` now routes the layer texture, layer blend,
+  and final-plane composite callbacks through local wrappers around the
+  retained helpers; the remaining work in this lane is now just branch
+  selection glue.
 - 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

@@ -568,6 +568,27 @@ static void execute_canvas_draw_merge_layer_composite(
         execution);
 }
 
+static void execute_canvas_draw_merge_layer_texture(
+    const pp::panopainter::LegacyCanvasDrawMergeTextureAlphaUniforms& uniforms,
+    const pp::panopainter::LegacyCanvasDrawMergeLayerTextureExecution& execution)
+{
+    pp::panopainter::execute_legacy_canvas_draw_merge_layer_texture(uniforms, execution);
+}
+
+static void execute_canvas_draw_merge_layer_blend(
+    const pp::panopainter::LegacyCanvasDrawMergeLayerBlendUniforms& uniforms,
+    const pp::panopainter::LegacyCanvasDrawMergeLayerBlendExecution& execution)
+{
+    pp::panopainter::execute_legacy_canvas_draw_merge_layer_blend(uniforms, execution);
+}
+
+static void execute_canvas_draw_merge_final_plane_composite(
+    const pp::panopainter::LegacyCanvasDrawMergeFinalPlaneCompositeUniforms& uniforms,
+    const pp::panopainter::LegacyCanvasDrawMergeFinalPlaneCompositeExecution& execution)
+{
+    pp::panopainter::execute_legacy_canvas_draw_merge_final_plane_composite(uniforms, execution);
+}
+
 glm::vec4 Canvas::stroke_draw_samples(
     int i,
     std::vector<vertex_t>& P,
@@ -1560,7 +1581,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                             });
                     },
                     .execute_layer_texture = [&] {
-                        pp::panopainter::execute_legacy_canvas_draw_merge_layer_texture(
+                        execute_canvas_draw_merge_layer_texture(
                     pp::panopainter::LegacyCanvasDrawMergeTextureAlphaUniforms {
                         .mvp = ortho,
                         .texture_slot = 0,
@@ -1584,7 +1605,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                     });
                     },
                     .execute_layer_blend = [&] {
-                        pp::panopainter::execute_legacy_canvas_draw_merge_layer_blend(
+                        execute_canvas_draw_merge_layer_blend(
                     pp::panopainter::LegacyCanvasDrawMergeLayerBlendUniforms {
                         .shader = {
                             .mvp = ortho,
@@ -1633,7 +1654,7 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
 
         if (use_blend)
         {
-            pp::panopainter::execute_legacy_canvas_draw_merge_final_plane_composite(
+            execute_canvas_draw_merge_final_plane_composite(
                 pp::panopainter::LegacyCanvasDrawMergeFinalPlaneCompositeUniforms {
                     .checkerboard = {
                         .mvp = ortho,