docs: update memory and threading API docs

docs/api/memory/linear-allocator/get-total-allocated.md

@@ -0,0 +1,36 @@
+# LinearAllocator::GetTotalAllocated
+
+```cpp
+size_t GetTotalAllocated() const override;
+```
+
+返回自分配器创建以来的累计分配字节数。对于线性分配器，这等于当前已使用的字节数（`m_offset`）。
+
+**参数：** 无
+
+**返回：** 累计已分配的字节数
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+#include <XCEngine/Memory/LinearAllocator.h>
+
+LinearAllocator allocator(1024);
+
+size_t total1 = allocator.GetTotalAllocated(); // 0
+
+allocator.Allocate(256);
+allocator.Allocate(128);
+
+size_t total2 = allocator.GetTotalAllocated(); // 384
+
+allocator.Clear();
+
+size_t total3 = allocator.GetTotalAllocated(); // 0（Clear 后重置）
+```
+
+## 相关文档
+
+- [LinearAllocator 总览](linear-allocator.md) - 返回类总览