Improve memory safety and alignment handling

- Updated `.gitignore` to include `.vscode/` and clarified comments.
- Introduced `SafeHandle` for managing memory alignment and safe access.
- Refactored `UnsafeArray<T>` to add bounds checking and alignment logic.
- Added `IUnsafeHashCollection<T>` for specialized hash-based collections.
- Refactored `UnsafeHashMap<TKey, TValue>` and `UnsafeHashSet<T>` to use `HashMapHelper<TKey>` with alignment support.
- Made `UnsafeSlotMap<T>` methods `readonly` for immutability.
- Enhanced `HashMapHelper<TKey>` with alignment-aware buffer management and validation.
- Updated benchmarks to use `UnsafeArray<Vector256<int>>` and added capacity checks.
- Incremented assembly version to `1.1.3` in `Misaki.HighPerformance.LowLevel.csproj`.
- Updated `Program.cs` to run `CollectionBenchmark` and demonstrate safe disposal handling.

Misaki.HighPerformance.LowLevel/Buffer/SafeHandle.cs

@@ -0,0 +1,31 @@
+namespace Misaki.HighPerformance.LowLevel.Buffer;
+
+public unsafe struct SafeHandle
+{
+    private const nuint _ALIGNMENT = 16u;
+
+    public int valid;
+
+    public static nuint GetAlignWithHeader(nuint baseAlign)
+    {
+        return Math.Max(_ALIGNMENT, baseAlign);
+    }
+
+    public static nuint GetPaddedHeaderSize(nuint baseAlign)
+    {
+        var headerBaseSize = (nuint)sizeof(SafeHandle);
+        var dataAlignment = Math.Max(_ALIGNMENT, baseAlign);
+        return (headerBaseSize + (dataAlignment - 1u)) & ~(dataAlignment - 1u);
+    }
+
+    public static SafeHandle* GetSafeHandle(void* ptr, nuint baseAlign)
+    {
+        if (ptr == null)
+        {
+            return null;
+        }
+
+        var alignedHeaderSize = GetPaddedHeaderSize(baseAlign);
+        return (SafeHandle*)((byte*)ptr - alignedHeaderSize);
+    }
+}