Implement movie mask transition opcode

engine/Age.Engine.Tests/MovieMaskSurfaceTests.cs

@@ -0,0 +1,55 @@
+using Age.Engine.Model;
+using Age.Engine.Sys4;
+using Xunit;
+
+public class MovieMaskSurfaceTests
+{
+    [Fact]
+    public void ExtractGreenUsesLogicalRowsAndRequestedMaskDimensions()
+    {
+        var frame = new RgbaImage(2, 2,
+        [
+            1, 10, 2, 255, 3, 20, 4, 255,
+            5, 30, 6, 255, 7, 40, 8, 255,
+        ]);
+
+        Assert.Equal(new byte[] { 10, 20, 99, 30, 40, 99 },
+            MovieMaskSurface.ExtractGreen(frame, 3, 2, 99));
+    }
+
+    [Fact]
+    public void ApplyPreservesRgbAndUsesExactPackedCarryAlpha()
+    {
+        var captured = new RgbaImage(2, 1,
+        [
+            0, 0, 0, 128,
+            255, 0, 0, 128,
+        ]);
+
+        RgbaImage result = MovieMaskSurface.Apply(captured, [255, 255], 2, 1, 0, 0);
+
+        Assert.Equal(new byte[]
+        {
+            0, 0, 0, 127,       // packed 0x80000000: native differs from alpha*255/255
+            255, 0, 0, 128,     // packed red carry reaches the output alpha byte
+        }, result.Pixels);
+        Assert.Equal(new byte[] { 0, 0, 0, 128, 255, 0, 0, 128 }, captured.Pixels);
+    }
+
+    [Fact]
+    public void ApplyClipsSignedRectangleAndLeavesOutsidePixelsUnchanged()
+    {
+        var captured = new RgbaImage(3, 1,
+        [
+            1, 2, 3, 255,
+            4, 5, 6, 255,
+            7, 8, 9, 255,
+        ]);
+
+        RgbaImage result = MovieMaskSurface.Apply(captured, [0, 64, 128], 3, 1, -1, 0);
+
+        Assert.Equal((byte)63, result.Pixels[3]);  // mask column 1
+        Assert.Equal((byte)127, result.Pixels[7]); // mask column 2
+        Assert.Equal((byte)255, result.Pixels[11]);
+    }
+}