Promote RND-006 compositor gpu task

docs/modernization/tasks.md

@@ -388,6 +388,34 @@ Completed Task Log:
 | --- | --- | ---: | --- | --- |
 | 2026-06-13 | RND-005 | +2 hardening and future backend readiness | `ctest --preset desktop-gpu --build-config Debug --output-on-failure`; `ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure` | `4a44e6cd` |
 
+### RND-006 - Add Desktop GPU Compositor Golden Gate
+
+Status: Ready
+Score: +2 hardening and future backend readiness
+Debt: `DEBT-0036`
+Scope: `tests/`, `CMakeLists.txt`, renderer test helpers only
+
+Goal:
+
+Create the next `desktop-gpu` readback gate for the compositor path so the
+OpenGL parity lane covers a second live render target, not just the existing
+clear/preview fixtures.
+
+Done Checks:
+
+- `ctest --preset desktop-gpu --build-config Debug` has a compositor readback
+  gate in addition to the existing GPU fixtures.
+- The test is skipped with a clear message when no GPU/context is available.
+- The roadmap and debt log describe the compositor output the gate covers and
+  what remains.
+
+Validation:
+
+```powershell
+ctest --preset desktop-gpu --build-config Debug --output-on-failure
+ctest --preset desktop-fast --build-config Debug -R "pp_paint_renderer_compositor|pp_paint_renderer_stroke_execution" --output-on-failure
+```
+
 ### PLT-001 - Split Apple Picker/Browse Service From Legacy Platform Adapter
 
 Status: Done