feat: implement descriptor heap with thread-safe allocation

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

docs/changelogs/README.md

@@ -16,6 +16,7 @@ changelogs/
 
 | 日期 | 提交 | 变更内容 |
 |------|------|----------|
+| 2026-03-30 | [描述符堆实现](./2026-03/20260330-d3d12-descriptor-heap.md) | D3D12 描述符堆管理和线程安全分配 |
 | 2026-03-27 | [Fence同步机制](./2026-03/20260327-d3d12-fence-sync.md) | D3D12 Fence CPU-GPU 帧同步实现 |
 | 2026-03-26 | [命令队列与多帧缓冲](./2026-03/20260326-d3d12-command-queue.md) | D3D12 命令队列和多帧渲染架构 |
 | 2026-03-26 | [D3D12设备初始化](./2026-03/20260326-d3d12-device-init.md) | D3D12 设备创建与调试层实现 |