Implement native ADV Hide Window path

engine/Age.Engine.Tests/CurDecoderTests.cs

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+using System.Linq;
+using Age.Engine.Sys4;
+using Xunit;
+
+public class CurDecoderTests
+{
+    [Fact]
+    public void HimegariCursor_DecodesPixelsAndHotspot()
+    {
+        var resources = ResourceMap.Load();
+        var entry = resources.ResolveCursor(0x3318);
+
+        Assert.NotNull(entry);
+        Assert.Equal("CURSOR03.CUR", entry!.Name);
+        var cursor = resources.DecodeCursor(entry);
+
+        Assert.Equal(32, cursor.Image.Width);
+        Assert.Equal(32, cursor.Image.Height);
+        Assert.Equal(25, cursor.HotspotX);
+        Assert.Equal(25, cursor.HotspotY);
+        Assert.Contains(cursor.Image.Pixels.Where((_, i) => i % 4 == 3), alpha => alpha == 0);
+        Assert.Contains(cursor.Image.Pixels.Where((_, i) => i % 4 == 3), alpha => alpha == 255);
+    }
+
+    [Fact]
+    public void TruncatedCursor_IsRejected()
+        => Assert.Throws<InvalidDataException>(() => CurDecoder.Decode(new byte[21], "bad.cur"));
+}