OpenGL: Add test documentation for minimal integration test

- Create TEST_SPEC.md with OpenGL test hierarchy and specifications
- Add section and inline comments to main.cpp explaining:
  - Window size calculation with AdjustWindowRect
  - glFinish before screenshot for GPU sync
  - Target frame count warm-up period
  - Shutdown order (reverse of initialization)

tests/RHI/OpenGL/integration/minimal/main.cpp

@@ -77,12 +77,14 @@ LRESULT CALLBACK WindowProc(HWND hwnd, UINT msg, WPARAM wParam, LPARAM lParam) {
 }
 
 int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine, int nShowCmd) {
+    // Initialize logger
     Logger::Get().Initialize();
     Logger::Get().AddSink(std::make_unique<ConsoleLogSink>());
     Logger::Get().SetMinimumLevel(LogLevel::Debug);
 
     Log("[INFO] OpenGL Integration Test Starting");
 
+    // Register window class
     WNDCLASSEXW wc = {};
     wc.cbSize = sizeof(WNDCLASSEXW);
     wc.style = CS_HREDRAW | CS_VREDRAW;
@@ -95,9 +97,11 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
         return -1;
     }
 
+    // Calculate full window size from client area size
     RECT rect = { 0, 0, gWidth, gHeight };
     AdjustWindowRect(&rect, WS_OVERLAPPEDWINDOW, FALSE);
 
+    // Create window
     HWND hwnd = CreateWindowExW(
         0,
         L"XCEngine_OpenGL_Test",
@@ -113,6 +117,7 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
         return -1;
     }
 
+    // Initialize OpenGL device with existing window
     OpenGLDevice device;
     if (!device.InitializeWithExistingWindow(hwnd)) {
         Log("[ERROR] Failed to initialize OpenGL device");
@@ -122,12 +127,15 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
     ShowWindow(hwnd, nShowCmd);
     UpdateWindow(hwnd);
 
+    // Log OpenGL device info
     Log("[INFO] OpenGL Device: %S", device.GetDeviceInfo().renderer.c_str());
     Log("[INFO] OpenGL Version: %S", device.GetDeviceInfo().version.c_str());
 
+    // Create swap chain for rendering
     OpenGLSwapChain swapChain;
     swapChain.Initialize(hwnd, gWidth, gHeight);
 
+    // Main render loop
     MSG msg = {};
     int frameCount = 0;
     const int targetFrameCount = 30;
@@ -140,21 +148,27 @@ int WINAPI WinMain(HINSTANCE hInstance, HINSTANCE hPrevInstance, LPSTR lpCmdLine
             TranslateMessage(&msg);
             DispatchMessageW(&msg);
         } else {
+            // Set viewport and clear color for each frame
             glViewport(0, 0, gWidth, gHeight);
             glClearColor(1.0f, 0.0f, 0.0f, 1.0f);
             glClear(GL_COLOR_BUFFER_BIT | GL_DEPTH_BUFFER_BIT);
+
+            // Present the rendered frame
             swapChain.Present(0, 0);
             frameCount++;
         }
     }
 
+    // Take screenshot after target frame count is reached
+    // glFinish ensures all OpenGL commands are completed before reading pixels
     Log("[INFO] Reached target frame count %d - taking screenshot!", targetFrameCount);
     SaveScreenshotPPM("minimal.ppm", gWidth, gHeight);
 
+    // Shutdown in reverse order of initialization
     swapChain.Shutdown();
     device.Shutdown();
     Logger::Get().Shutdown();
 
     Log("[INFO] OpenGL Integration Test Finished");
     return 0;
-}
+}