feat(buffer)!: refactor allocators to use MemoryPool<T>

Refactor memory allocation system to use generic MemoryPool<TAllocator, TOpts> for arena, stack, and free list allocators, replacing custom allocator structs. Introduce MemoryBlock as a safer, more robust replacement for UnTypedArray. Improve thread safety, safety checks, and documentation. Reorder and clarify Allocator enum. Add comprehensive unit tests for all allocators and pointer assertion utilities. Update project to enable safety checks in Debug builds. Remove obsolete interfaces and ensure consistent deallocation with MemoryUtility.Free.

BREAKING CHANGE: Custom allocator structs are removed and replaced with MemoryPool-based abstraction. UnTypedArray is replaced by MemoryBlock. Allocator enum order and semantics are changed. Public API changes may require code updates.

Misaki.HighPerformance.LowLevel/Collections/Contracts/IUnsafeCollection.cs

@@ -78,18 +78,4 @@ public interface IUnsafeHashCollection<T> : IEnumerable<T>, IDisposable
     /// <param name="newSize">Specifies the new size to which the collection should be adjusted.</param>
     /// <param name="option">Specifies allocation options that may affect how memory is managed during the resize operation.</param>
     void Resize(int newSize, AllocationOption option);
-}
-
-public interface IUnTypedCollection : IUnsafeCollection
-{
-    /// <summary>
-    /// The total size of the buffer in bytes.
-    /// </summary>
-    nuint Size
-    {
-        get;
-    }
-
-    ref T GetElementAt<T>(nuint index)
-        where T : unmanaged;
 }