docs: update memory and threading API docs

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

@@ -0,0 +1,33 @@
+# LinearAllocator::GetPeakAllocated
+
+```cpp
+size_t GetPeakAllocated() const override;
+```
+
+返回自分配器创建以来的峰值分配字节数。对于线性分配器，这等于总容量（`m_capacity`）。
+
+**参数：** 无
+
+**返回：** 峰值分配字节数（等于容量）
+
+**复杂度：** O(1)
+
+**示例：**
+
+```cpp
+#include <XCEngine/Memory/LinearAllocator.h>
+
+LinearAllocator allocator(1024);
+
+size_t peak = allocator.GetPeakAllocated(); // 1024（容量）
+
+allocator.Allocate(512);
+size_t used = allocator.GetUsedSize(); // 512
+
+// 即使未达到容量，峰值仍为容量大小
+size_t peak_after = allocator.GetPeakAllocated(); // 1024
+```
+
+## 相关文档
+
+- [LinearAllocator 总览](linear-allocator.md) - 返回类总览