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/Buffer/Stack.cs

@@ -1,4 +1,4 @@
-using System.Diagnostics;
+using System.Collections;
 using System.Runtime.CompilerServices;
 
 namespace Misaki.HighPerformance.LowLevel.Buffer;
@@ -8,14 +8,14 @@ namespace Misaki.HighPerformance.LowLevel.Buffer;
 /// blocks within a preallocated buffer.
 /// </summary>
 /// <remarks>This is not a thread-safe implementation.</remarks>
-public unsafe partial struct Stack : IMemoryAllocator<Stack, Stack.CreationOpts>
+public unsafe partial struct Stack : IMemoryAllocator<Stack, Stack.CreationOptions>
 {
-    public struct CreationOpts
+    public struct CreationOptions
     {
         public nuint size;
     }
 
-    public static Stack Create(in CreationOpts opts)
+    public static Stack Create(in CreationOptions opts)
     {
         return new Stack(opts.size);
     }
@@ -57,12 +57,9 @@ public unsafe partial struct Stack : IMemoryAllocator<Stack, Stack.CreationOpts>
     private uint _activeScopeCount;
 #endif
 
-    internal readonly byte* Buffer => _buffer;
-    internal nuint Offset
-    {
-        readonly get => _offset;
-        set => _offset = value;
-    }
+    public readonly byte* Buffer => _buffer;
+    public readonly nuint Size => _size;
+    public readonly nuint Offset => _offset;
 
     /// <summary>
     /// Initializes a new instance of the StackAllocator class with a buffer of the specified size.
@@ -141,6 +138,44 @@ public unsafe partial struct Stack : IMemoryAllocator<Stack, Stack.CreationOpts>
         return ptr;
     }
 
+    public void* Reallocate(void* ptr, nuint oldSize, nuint newSize, nuint alignment, AllocationOption allocationOption)
+    {
+        if (_buffer == null)
+        {
+            return null;
+        }
+
+        if (ptr == null)
+        {
+            return Allocate(newSize, alignment, allocationOption);
+        }
+
+        var oldBase = _buffer + _offset - oldSize;
+        if (ptr == oldBase)
+        {
+            if (newSize > oldSize)
+            {
+                var diff = newSize - oldSize;
+                _offset += diff;
+                if (allocationOption.HasFlag(AllocationOption.Clear))
+                {
+                    MemClear(_buffer + _offset - diff, diff);
+                }
+            }
+
+            return ptr;
+        }
+
+        var newPtr = Allocate(newSize, alignment, allocationOption);
+        if (newPtr == null)
+        {
+            return null;
+        }
+
+        MemCpy(newPtr, ptr, Math.Min(oldSize, newSize));
+        return newPtr;
+    }
+
     [MethodImpl(MethodImplOptions.AggressiveInlining)]
     public readonly void Free(void* ptr)
     {