feat: Add method to check version in the overlay API

Author: ResurrectedTrader

src/D2SSharp/Model/D2SaveOverlay.cs

@@ -20,6 +20,19 @@ public struct HeaderLayout
     [FieldOffset(12)] public uint Checksum;
 
     internal readonly bool IsValid => Magic == D2Save.Magic;
+
+    /// <summary>
+    /// Returns the save file version from raw bytes without allocating.
+    /// </summary>
+    /// <param name="data">The save file data (minimum 8 bytes).</param>
+    /// <returns>The save file version.</returns>
+    public static uint GetVersion(ReadOnlySpan<byte> data)
+    {
+        ref readonly var header = ref MemoryMarshal.AsRef<HeaderLayout>(data);
+        if (!header.IsValid)
+            throw new InvalidDataException($"Invalid save file magic: 0x{header.Magic:X8}, expected 0x{D2Save.Magic:X8}");
+        return header.Version;
+    }
 }
 
 /// <summary>