D3D12 Resources:
- Add descriptor_handle struct with CPU/GPU handles
- Add descriptor_heap class for descriptor management
- Implement allocate() and free() methods
- Add mutex for thread-safe access
- Support all D3D12 descriptor heap types
D3D12 Core:
- Add device() function to expose main device
- Add release() template function for COM objects
Documentation:
- Add changelog for descriptor heap implementation
- Update D3D12 Wiki with descriptor heap section
- Mark descriptor heap task as completed
