Improve FreeList/TLSF allocators: alignment, GC, decommit

- FreeList: enforce min 16B alignment, use GCHandle for SharedState lifetime, switch to AllocZeroed, and use MemoryUtility for oversized allocs
- Add FreeList.CollectLocal() to flush thread-local caches
- TLSF: add decommitted flag, support front splitting for alignment, add Collect() to decommit large free blocks, use Munmap for cleanup
- Add VirtualMemoryBlock for virtual memory management
- Add tests for CollectLocal (FreeList) and Collect (TLSF)
- Update default allocator config and minor .csproj cleanup

Misaki.HighPerformance.LowLevel/Buffer/VirtualMemoryBlock.cs

@@ -0,0 +1,35 @@
+using Misaki.HighPerformance.LowLevel.Utilities;
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Misaki.HighPerformance.LowLevel.Buffer;
+
+public unsafe struct VirtualMemoryBlock : IDisposable
+{
+    private byte* _baseAddress;
+    private nuint _size;
+    private nuint _committed;
+
+    public VirtualMemoryBlock(nuint size)
+    {
+        _baseAddress = (byte*)MemoryUtility.Mmap(null, size, VirtualAllocationFlags.Reserve);
+        _size = size;
+        _committed = 0;
+    }
+
+    public void Dispose()
+    {
+        if (_baseAddress == null)
+        {
+            return;
+        }
+
+        var addr = _baseAddress;
+        _baseAddress = null;
+        _size = 0;
+        _committed = 0;
+
+        MemoryUtility.Munmap(addr, _size);
+    }
+}