Route VR checkerboard 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. Desktop VR checkerboard background
+  shader setup now routes through `legacy_canvas_draw_merge_services.h`; VR
+  still owns render state transitions, per-plane transforms, and draw
+  execution.
 - 2026-06-13: DEBT-0036 was narrowed again. `CanvasModeMaskCut::apply`
   retained `CompDraw` shader setup now routes through
   `legacy_canvas_stroke_composite_services.h`; mask-cut still owns render-task

docs/modernization/roadmap.md

@@ -3021,6 +3021,9 @@ 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.
+- 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.
 - `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/app_vr.cpp

@@ -3,6 +3,7 @@
 #include <cstdint>
 
 #include "app.h"
+#include "legacy_canvas_draw_merge_services.h"
 #include "legacy_canvas_stroke_composite_services.h"
 #include "legacy_canvas_stroke_erase_services.h"
 #include "legacy_ui_gl_dispatch.h"
@@ -274,9 +275,11 @@ void App::vr_draw(const glm::mat4& proj, const glm::mat4& camera, const glm::mat
             canvas->m_canvas->m_plane_transform[plane_index] *
             glm::translate(glm::vec3(0, 0, -1));
 
-        ShaderManager::use(kShader::Checkerboard);
-        ShaderManager::u_int(kShaderUniform::Colorize, false);
-        ShaderManager::u_mat4(kShaderUniform::MVP, plane_mvp);
+        pp::panopainter::setup_legacy_canvas_draw_merge_checkerboard_shader(
+            pp::panopainter::LegacyCanvasDrawMergeCheckerboardUniforms {
+                .mvp = plane_mvp,
+                .colorize = false,
+            });
         m_face_plane.draw_fill();
     }