feat(allocator): add unified Reallocate to all allocators

Introduce a unified Reallocate method to all memory allocator types (Arena, Stack, FreeList, VirtualArena, VirtualStack, DynamicArena) and require it in the IMemoryAllocator interface. This enables efficient resizing of memory blocks, with fast-path optimizations for stack-like allocators.

Update AllocationManager and MemoryPool to use the new Reallocate method, simplifying and optimizing memory resizing logic. Add public properties for buffer pointers, sizes, and offsets to allocator structs for easier diagnostics.

Set FreeList's default concurrency level to 1 and make its allocation method return null on dispose instead of throwing. Clean up vector types for formatting, fix UnsafeList's RemoveRangeSwapBack logic, and simplify RemoveAtSwapBack.

Simplify Program.cs to only run SPMDBenchmark. Add new unit tests for FixedString, UnsafeList, UnsafeHashMap, and UnsafeHashSet. Apply minor test code cleanups for consistency in TestUnsafeQueue.

BREAKING CHANGE: IMemoryAllocator now requires a Reallocate method, and allocator APIs have changed accordingly.

Misaki.HighPerformance.LowLevel/Collections/UnsafeList.cs

@@ -454,19 +454,19 @@ public unsafe struct UnsafeList<T> : IUnsafeCollection<T>
             return;
         }
 
-        var copyFrom = Math.Min(_count - length, start + length);
-        MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), start),
-            UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), copyFrom),
-            (uint)((_count - copyFrom) * sizeof(T))
-        );
+        var numToCopy = Math.Min(length, _count - (start + length));
+        var copyFrom = _count - numToCopy;
+
+        if (numToCopy > 0)
+        {
+            MemCpy(UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), start),
+                UnsafeUtility.ReadArrayElementUnsafe<T>(_array.GetUnsafePtr(), copyFrom),
+                (uint)((_count - copyFrom) * sizeof(T)));
+        }
+
         _count -= length;
     }
 
-    /// <summary>
-    /// Removes the element at the specified index by swapping it with the last element and reducing the collection
-    /// size.
-    /// </summary>
-    /// <param name="index">The zero-based index of the element to remove. Must be within the bounds of the collection.</param>
     public void RemoveAtSwapBack(int index)
     {
         RemoveRangeSwapBack(index, 1);