Extract Canvas draw_merge layer texture helper

docs/modernization/debt.md

@@ -18,6 +18,11 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge()` now routes
+  the standard per-layer texture-alpha pass through
+  `execute_legacy_canvas_draw_merge_layer_texture(...)`; per-plane iteration,
+  temporary-stroke branch selection, and concrete layer RTT ownership remain 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/roadmap.md

@@ -67,6 +67,9 @@ Recent 2026-06-13 retained preview reductions continue to narrow DEBT-0036:
 feedback/material/composite planning and stroke-shader uniform assembly through
 `plan_legacy_node_stroke_preview_pass_orchestration(...)`, leaving the preview
 node with a smaller live-GL callback surface around pass execution.
+`Canvas::draw_merge()` also now routes its standard per-layer texture-alpha pass
+through `execute_legacy_canvas_draw_merge_layer_texture(...)`, reducing another
+piece of retained layer-composite sequencing.
 
 ## Target Component Architecture
 

docs/modernization/tasks.md

@@ -509,6 +509,13 @@ Done Checks:
 
 Progress Notes:
 
+- 2026-06-13: `Canvas::draw_merge()` now routes the standard per-layer
+  texture-alpha pass through
+  `execute_legacy_canvas_draw_merge_layer_texture(...)`; temporary-stroke
+  branches, per-plane iteration, and concrete RTT ownership remain local to
+  `Canvas`. Next slice should target another narrow draw-merge execution seam
+  without reopening landed temporary-composite, layer-blend, final-plane, or
+  texture-alpha helpers.
 - 2026-06-13: `NodeStrokePreview::draw_stroke_immediate()` now routes retained
   preview feedback/material/composite planning plus stroke-shader uniform
   assembly through `plan_legacy_node_stroke_preview_pass_orchestration(...)`;

src/canvas.cpp

@@ -1518,19 +1518,28 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
             }
             else
             {
-                m_cam_fov < 20.f ? m_sampler_nearest.bind(0) : m_sampler.bind(0);
-                pp::panopainter::setup_legacy_canvas_draw_merge_texture_alpha_shader(
+                pp::panopainter::execute_legacy_canvas_draw_merge_layer_texture(
                     pp::panopainter::LegacyCanvasDrawMergeTextureAlphaUniforms {
                         .mvp = ortho,
                         .texture_slot = 0,
                         .alpha = m_layers[layer_index]->m_opacity,
                         .highlight = m_layers[layer_index]->m_hightlight,
+                    },
+                    pp::panopainter::LegacyCanvasDrawMergeLayerTextureExecution {
+                        .bind_sampler = [&] {
+                            m_cam_fov < 20.f ? m_sampler_nearest.bind(0) : m_sampler.bind(0);
+                        },
+                        .bind_layer_texture = [&] {
+                            set_active_texture_unit(0);
+                            m_layers[layer_index]->rtt(plane_index).bindTexture();
+                        },
+                        .draw = [&] {
+                            m_plane.draw_fill();
+                        },
+                        .unbind_layer_texture = [&] {
+                            m_layers[layer_index]->rtt(plane_index).unbindTexture();
+                        },
                     });
-
-                set_active_texture_unit(0);
-                m_layers[layer_index]->rtt(plane_index).bindTexture();
-                m_plane.draw_fill();
-                m_layers[layer_index]->rtt(plane_index).unbindTexture();
             }
 
             if (use_blend)

src/legacy_canvas_draw_merge_services.h

@@ -80,6 +80,13 @@ struct LegacyCanvasDrawMergeLayerBlendExecution {
     std::function<void()> unbind_merge_texture;
 };
 
+struct LegacyCanvasDrawMergeLayerTextureExecution {
+    std::function<void()> bind_sampler;
+    std::function<void()> bind_layer_texture;
+    std::function<void()> draw;
+    std::function<void()> unbind_layer_texture;
+};
+
 struct LegacyCanvasDrawMergeFinalPlaneCompositeUniforms {
     LegacyCanvasDrawMergeCheckerboardUniforms checkerboard;
     LegacyCanvasDrawMergeTextureUniforms texture;
@@ -273,6 +280,17 @@ inline void execute_legacy_canvas_draw_merge_layer_blend(
     execution.unbind_merge_texture();
 }
 
+inline void execute_legacy_canvas_draw_merge_layer_texture(
+    const LegacyCanvasDrawMergeTextureAlphaUniforms& uniforms,
+    const LegacyCanvasDrawMergeLayerTextureExecution& execution)
+{
+    execution.bind_sampler();
+    setup_legacy_canvas_draw_merge_texture_alpha_shader(uniforms);
+    execution.bind_layer_texture();
+    execution.draw();
+    execution.unbind_layer_texture();
+}
+
 inline void execute_legacy_canvas_draw_merge_final_plane_composite(
     const LegacyCanvasDrawMergeFinalPlaneCompositeUniforms& uniforms,
     const LegacyCanvasDrawMergeFinalPlaneCompositeExecution& execution)