Refactor memory management and improve allocation APIs

Updated `ReallocFunc` to support `oldSize`, `newSize`, and `AllocationOption`, enabling more granular control over memory reallocation. Simplified `AllocationInfo` by removing the `FreeHandler` property. Enhanced `Reallocate` and `Allocate` methods in `AllocationManager` to handle memory clearing and tracking more effectively.

Refactored `UnsafeSparseSet` to use `UnsafeArray<T>` directly, added a `generations` array, and replaced the `freeList` with `UnsafeStack<int>` for better performance and simplicity. Updated `Resize`, `Add`, and `Remove` methods to improve memory handling and code clarity.

Introduced `AllocationOption` support in `Resize` methods across `IUnsafeCollection` implementations for flexible resizing behavior. Added `GetUnsafePtr` extension methods in `UnsafeUtilities` for direct access to span data.

Added new tests for `UnsafeSparseSet` to validate resizing, clearing, enumeration, and memory compaction. These changes improve memory management, enhance performance, and ensure correctness.

Misaki.HighPerformance.LowLevel/Utilities/UnsafeUtilities.cs

@@ -156,4 +156,34 @@ public static unsafe class UnsafeUtilities
     {
         return new UnsafeArray<TOut>((TOut*)array.GetUnsafePtr(), array.Count * sizeof(TIn) / sizeof(TOut));
     }
+
+    /// <summary>
+    /// Returns a pointer to the first element of the specified span. This method enables direct, unsafe access to the underlying data of the span.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the span. Must be an unmanaged type.</typeparam>
+    /// <param name="span">The span whose underlying data pointer is to be obtained.</param>
+    /// <returns>A pointer to the first element of the span. If the span is empty, the returned pointer is undefined and must not be dereferenced.</returns>
+    public static T* GetUnsafePtr<T>(this Span<T> span)
+        where T : unmanaged
+    {
+        fixed (T* ptr = span)
+        {
+            return ptr;
+        }
+    }
+
+    /// <summary>
+    /// Returns a pointer to the first element of the specified span. This method enables direct, unsafe access to the underlying data of the span.
+    /// </summary>
+    /// <typeparam name="T">The type of elements in the span. Must be an unmanaged type.</typeparam>
+    /// <param name="span">The span whose underlying data pointer is to be obtained.</param>
+    /// <returns>A pointer to the first element of the span. If the span is empty, the returned pointer is undefined and must not be dereferenced.</returns>
+    public static T* GetUnsafePtr<T>(this ReadOnlySpan<T> span)
+        where T : unmanaged
+    {
+        fixed (T* ptr = span)
+        {
+            return ptr;
+        }
+    }
 }