feat(memory): improve oversized alloc handling & cleanup

Enhanced FreeList to handle oversized allocations by bypassing chunk management and allocating directly from the OS, with proper header assignment and immediate release on free. Updated IUnsafeCollection<T> documentation, removed unnecessary array clearing in Clear() methods, refined UnsafeSlotMap initialization logic, and removed redundant default element addition. Bumped assembly version to 1.5.1.

Misaki.HighPerformance.LowLevel/Collections/UnsafeSlotMap.cs

@@ -132,8 +132,8 @@ public unsafe struct UnsafeSlotMap<T> : IUnsafeCollection<T>
         if (!allocationOption.HasFlag(AllocationOption.Clear))
         {
             _generations.AsSpan().Clear();
+            _validBits.ClearAll();
         }
-        _validBits.ClearAll();
 
         _count = 0;
         _capacity = capacity;
@@ -324,8 +324,6 @@ public unsafe struct UnsafeSlotMap<T> : IUnsafeCollection<T>
         _validBits.ClearAll();
 
         _count = 0;
-
-        Add(default, out _);
     }
 
     public readonly void* GetUnsafePtr()