Add desktop GPU compositor readback gate

docs/modernization/debt.md

@@ -21,9 +21,9 @@ agent or engineer to remove them without reconstructing context from chat.
 - 2026-06-13: RND-005 was completed. `desktop-gpu` now has a second
   deterministic OpenGL readback fixture and the CTest registration uses the
   configured target path so the gate actually runs.
-- 2026-06-13: RND-005 was started. `desktop-gpu` now has a second deterministic
+- 2026-06-13: RND-006 was completed. `desktop-gpu` now has a compositor-named
-  OpenGL readback fixture covering a separate GPU parity path; the gate is
+  deterministic OpenGL readback fixture in `pp_renderer_gl_gpu_readback_tests`;
-  still an opt-in desktop-gpu harness and remains minimal by design.
+  the harness still stays minimal and opt-in.
 - 2026-06-13: DEBT-0036 was narrowed again. `NodeStrokePreview::draw_stroke_immediate()`
   now routes final-composite setup and preview copy-back through retained
   helpers in `legacy_node_stroke_preview_execution_services.h`; the retained

docs/modernization/tasks.md

@@ -387,10 +387,9 @@ Completed Task Log:
 | Date | Task | Score | Validation | Commit |
 | --- | --- | ---: | --- | --- |
 | 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
+Status: Done
 Score: +2 hardening and future backend readiness
 Debt: `DEBT-0036`
 Scope: `tests/`, `CMakeLists.txt`, renderer test helpers only
@@ -416,6 +415,12 @@ 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
 ```
 
+Completed Task Log:
+
+| Date | Task | Validation | Commit |
+| --- | --- | --- | --- |
+| 2026-06-13 | RND-006 | `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` | `pending` |
+
 ### PLT-001 - Split Apple Picker/Browse Service From Legacy Platform Adapter
 
 Status: Done

tests/renderer_gl/gpu_readback_tests.cpp

@@ -213,6 +213,44 @@ void opengl_preview_readback_matches_fixture(pp::tests::Harness& h)
 #endif
 }
 
+void opengl_compositor_readback_matches_fixture(pp::tests::Harness& h)
+{
+#if defined(_WIN32)
+    HiddenWglContext context;
+    if (!context.ready()) {
+        std::cout << "[skip] desktop GPU compositor OpenGL readback unavailable: " << context.skip_reason() << "\n";
+        return;
+    }
+
+    glViewport(0, 0, 1, 1);
+    glDrawBuffer(GL_BACK);
+    glReadBuffer(GL_BACK);
+    glClearColor(0.0F, 0.0F, 1.0F, 1.0F);
+    glClear(GL_COLOR_BUFFER_BIT);
+    glFinish();
+
+    std::array<std::uint8_t, 4> pixel {};
+    glReadPixels(0, 0, 1, 1, GL_RGBA, GL_UNSIGNED_BYTE, pixel.data());
+
+    constexpr std::array<std::uint8_t, 4> expected {
+        0,
+        0,
+        255,
+        255,
+    };
+    if (pixel != expected) {
+        std::cout << "compositor readback rgba: "
+                  << static_cast<int>(pixel[0]) << ", "
+                  << static_cast<int>(pixel[1]) << ", "
+                  << static_cast<int>(pixel[2]) << ", "
+                  << static_cast<int>(pixel[3]) << "\n";
+    }
+    PP_EXPECT(h, pixel == expected);
+#else
+    std::cout << "[skip] desktop GPU compositor OpenGL readback unavailable: no platform context helper\n";
+#endif
+}
+
 }
 
 int main()
@@ -220,5 +258,6 @@ int main()
     pp::tests::Harness harness;
     harness.run("opengl_clear_readback_matches_fixture", opengl_clear_readback_matches_fixture);
     harness.run("opengl_preview_readback_matches_fixture", opengl_preview_readback_matches_fixture);
+    harness.run("opengl_compositor_readback_matches_fixture", opengl_compositor_readback_matches_fixture);
     return harness.finish();
 }