- Add ShaderLanguage enum (HLSL, GLSL, SPIRV)
- Extend ShaderCompileDesc with source/sourceLanguage fields
- D3D12Device::CompileShader supports both file and embedded source
- OpenGLDevice::CompileShader supports embedded GLSL source
- Refactor test_shader.cpp to use embedded source for both backends
This enables consistent shader compilation across D3D12 and OpenGL
backends while maintaining backend-specific shader language support.
- Add RHITestFixture with RHI_BACKEND env var support for backend selection
- Add unit tests for: Device, Buffer, Texture, SwapChain, CommandList, CommandQueue, Shader, Fence, Sampler
- Tests can run against D3D12 or OpenGL backends via RHI_BACKEND env var
- Add integration folder placeholder for future integration tests
