refactor: reorganize unit tests into separate folder

- Move test_*.cpp and fixtures/ to tests/RHI/D3D12/unit/
- Create unit/CMakeLists.txt with proper test configuration
- Simplify parent CMakeLists.txt to use add_subdirectory
- Integration tests remain in integration/ folder

tests/RHI/D3D12/unit/test_shader.cpp

@@ -0,0 +1,20 @@
+#include "fixtures/D3D12TestFixture.h"
+
+TEST_F(D3D12TestFixture, Shader_Placeholder) {
+    ASSERT_NE(GetDevice(), nullptr);
+}
+
+TEST_F(D3D12TestFixture, Shader_GetVertexShaderProfile) {
+    const char* profile = "vs_6_0";
+    EXPECT_STREQ(profile, "vs_6_0");
+}
+
+TEST_F(D3D12TestFixture, Shader_GetPixelShaderProfile) {
+    const char* profile = "ps_6_0";
+    EXPECT_STREQ(profile, "ps_6_0");
+}
+
+TEST_F(D3D12TestFixture, Shader_GetComputeShaderProfile) {
+    const char* profile = "cs_6_0";
+    EXPECT_STREQ(profile, "cs_6_0");
+}