Route draw merge blend setup through helper

docs/modernization/debt.md

@@ -18,6 +18,12 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_merge` non-stroke
+  `TextureBlend` shader setup now routes through
+  `legacy_canvas_draw_merge_services.h`, preserving the optional `TexBG`
+  uniform only when copy-based blend destination feedback is active. Canvas
+  still owns draw-merge layer iteration, framebuffer copies, sampler/texture
+  binding, and draw ordering.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeCanvas` non-stroke
   `TextureBlend` shader setup now routes through
   `legacy_canvas_draw_merge_services.h`, including the optional destination

docs/modernization/roadmap.md

@@ -2992,6 +2992,11 @@ Results:
   checkerboard backgrounds and the final merged texture redraw over the grid;
   layer traversal, blend destination copies, sampler/texture binding, and draw
   ordering remain in the legacy Canvas path.
+- `Canvas::draw_merge` non-stroke layer blending now shares the retained
+  draw-merge shader setup helper for `TextureBlend` uniforms, preserving the
+  optional `TexBG` uniform when copy-based blend destination feedback is active.
+  Layer traversal, framebuffer copies, sampler/texture binding, and draw
+  ordering remain in the legacy Canvas path.
 - `Canvas::stroke_draw_mix` now shares the retained stroke composite shader
   helper for mixer-pass `CompDraw` setup, while preserving its caller-specific
   texture slot uniforms. Mixer framebuffer/scissor state, sampler and texture

src/canvas.cpp

@@ -1411,15 +1411,18 @@ void Canvas::draw_merge(bool draw_checkerboard, std::array<bool, 6> faces /*= SI
                 m_sampler.bind(0);
                 m_sampler.bind(2);
 
-                ShaderManager::use(kShader::TextureBlend);
-                ShaderManager::u_int(kShaderUniform::Tex, 0);
-                ShaderManager::u_int(kShaderUniform::BlendMode, m_layers[layer_index]->m_blend_mode);
-                ShaderManager::u_float(kShaderUniform::Alpha, 1.f);
-                ShaderManager::u_mat4(kShaderUniform::MVP, ortho);
+                pp::panopainter::setup_legacy_canvas_draw_merge_texture_blend_shader(
+                    pp::panopainter::LegacyCanvasDrawMergeTextureBlendUniforms {
+                        .mvp = ortho,
+                        .texture_slot = 0,
+                        .destination_texture_slot = 2,
+                        .use_destination_texture = copy_blend_destination,
+                        .blend_mode = m_layers[layer_index]->m_blend_mode,
+                        .alpha = 1.f,
+                    });
                 if (copy_blend_destination)
                 {
                     m_sampler.bind(2);
-                    ShaderManager::u_int(kShaderUniform::TexBG, 2);
                 }
 
                 set_active_texture_unit(0);