Route object composite texture setup through helper

docs/modernization/debt.md

@@ -18,6 +18,10 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 2026-06-13: DEBT-0036 was narrowed again. `Canvas::draw_objects` retained
+  object composite `Texture` shader setup now routes through
+  `legacy_canvas_draw_merge_services.h`; draw_objects still owns temporary
+  face readback, sampler/texture binding, draw execution, and dirty-box updates.
 - 2026-06-13: DEBT-0036 was narrowed again. `Canvas::thumbnail_generate`
   thumbnail layer `TextureBlend` shader setup now routes through
   `legacy_canvas_draw_merge_services.h`; thumbnail generation still owns

docs/modernization/roadmap.md

@@ -3021,6 +3021,10 @@ Results:
   stroke composite shader setup helper, while render-task ordering, framebuffer
   copy bounds, sampler/texture binding, and draw execution remain in retained
   canvas-mode code.
+- `Canvas::draw_objects` retained object compositing now shares the retained
+  draw-merge texture shader setup helper, while temporary face readback,
+  sampler/texture binding, draw execution, and dirty-box updates remain in
+  retained Canvas code.
 - Desktop VR checkerboard background drawing now shares the retained draw-merge
   shader setup helper, while render state transitions, per-plane transforms,
   and draw execution remain in retained VR code.

src/canvas.cpp

@@ -3199,9 +3199,11 @@ void Canvas::draw_objects(std::function<void(const glm::mat4& camera, const glm:
             // draw the tmp layer into the actual layer
             if (has_data)
             {
-                ShaderManager::use(kShader::Texture);
-                ShaderManager::u_int(kShaderUniform::Tex, 0);
-                ShaderManager::u_mat4(kShaderUniform::MVP, glm::ortho(-0.5f, 0.5f, -0.5f, 0.5f));
+                pp::panopainter::setup_legacy_canvas_draw_merge_texture_shader(
+                    pp::panopainter::LegacyCanvasDrawMergeTextureUniforms {
+                        .mvp = glm::ortho(-0.5f, 0.5f, -0.5f, 0.5f),
+                        .texture_slot = 0,
+                    });
                 set_active_texture_unit(0);
                 m_sampler_nearest.bind(0);
                 rtt.bindTexture();