docs: update memory and threading API docs

docs/api/memory/linear-allocator/get-allocation-count.md

@@ -0,0 +1,31 @@
+# LinearAllocator::GetAllocationCount
+
+```cpp
+size_t GetAllocationCount() const override;
+```
+
+返回自分配器创建以来的累计分配次数。线性分配器不支持单个块释放，始终返回 0。
+
+**参数：** 无
+
+**返回：** 始终返回 `0`
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+#include <XCEngine/Memory/LinearAllocator.h>
+
+LinearAllocator allocator(1024);
+
+allocator.Allocate(256);
+allocator.Allocate(128);
+allocator.Allocate(64);
+
+size_t count = allocator.GetAllocationCount(); // 0（不支持计数）
+```
+
+## 相关文档
+
+- [LinearAllocator 总览](linear-allocator.md) - 返回类总览