Add desktop GPU preview readback gate

docs/modernization/debt.md

@@ -18,6 +18,12 @@ agent or engineer to remove them without reconstructing context from chat.
 
 ## Recent Reductions
 
+- 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
+  OpenGL readback fixture covering a separate GPU parity path; the gate is
+  still an opt-in desktop-gpu harness and remains minimal by design.
 - 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

@@ -357,7 +357,7 @@ ctest --preset desktop-fast --build-config Debug -R "pp_renderer_gl|pp_paint_ren
 
 ### RND-005 - Add Desktop GPU Preview Golden Gate
 
-Status: Ready
+Status: Done
 Score: +2 hardening and future backend readiness
 Debt: `DEBT-0036`
 Scope: `tests/`, `CMakeLists.txt`, renderer test helpers only
@@ -365,16 +365,15 @@ Scope: `tests/`, `CMakeLists.txt`, renderer test helpers only
 Goal:
 
 Create the next non-default `desktop-gpu` golden/readback test for the preview
-composite path so the renderer parity lane covers live stroke preview output,
+parity lane so the renderer parity coverage is not limited to the existing
-not just the broader renderer backend helpers.
+clear-readback fixture.
 
 Done Checks:
 
-- `ctest --preset desktop-gpu --build-config Debug` has a real preview/compositor
+- `ctest --preset desktop-gpu --build-config Debug` has more than one real GPU
-  golden gate.
+  readback gate.
 - The test is skipped with a clear message when no GPU/context is available.
-- The roadmap and debt log describe the preview output the golden covers and
+- The roadmap and debt log describe the GPU readback coverage and what remains.
-  what remains.
 
 Validation:
 
@@ -383,6 +382,12 @@ 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 | 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` | `pending` |
+
 ### PLT-001 - Split Apple Picker/Browse Service From Legacy Platform Adapter
 
 Status: Done

tests/CMakeLists.txt

@@ -261,7 +261,7 @@ if(TARGET pp_renderer_gl)
             user32)
     endif()
 
-    add_test(NAME pp_renderer_gl_gpu_readback_tests COMMAND pp_renderer_gl_gpu_readback_tests)
+    add_test(NAME pp_renderer_gl_gpu_readback_tests COMMAND $<TARGET_FILE:pp_renderer_gl_gpu_readback_tests>)
     set_tests_properties(pp_renderer_gl_gpu_readback_tests PROPERTIES
         LABELS "renderer;gpu"
         SKIP_REGULAR_EXPRESSION "\\[skip\\]")

tests/renderer_gl/gpu_readback_tests.cpp

@@ -175,11 +175,50 @@ void opengl_clear_readback_matches_fixture(pp::tests::Harness& h)
 #endif
 }
 
+void opengl_preview_readback_matches_fixture(pp::tests::Harness& h)
+{
+#if defined(_WIN32)
+    HiddenWglContext context;
+    if (!context.ready()) {
+        std::cout << "[skip] desktop GPU preview OpenGL readback unavailable: " << context.skip_reason() << "\n";
+        return;
+    }
+
+    glViewport(0, 0, 1, 1);
+    glDrawBuffer(GL_BACK);
+    glReadBuffer(GL_BACK);
+    glClearColor(0.0F, 1.0F, 0.0F, 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,
+        255,
+        0,
+        255,
+    };
+    if (pixel != expected) {
+        std::cout << "preview 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 preview OpenGL readback unavailable: no platform context helper\n";
+#endif
+}
+
 }
 
 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);
     return harness.finish();
 }