feat: add BlendMath (colorkey match + ARGB unpack)

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

engine/Age.Engine.Tests/BlendMathTests.cs

@@ -0,0 +1,34 @@
+using Age.Engine.Model;
+using Xunit;
+
+public class BlendMathTests
+{
+    [Fact]
+    public void NegativeColorKey_IsNoKey()
+    {
+        Assert.False(BlendMath.HasColorKey(-1));
+        Assert.False(BlendMath.ColorKeyMatches(0, 0, 0, -1));   // never matches when no key
+    }
+
+    [Fact]
+    public void ZeroColorKey_KeysBlack()
+    {
+        Assert.True(BlendMath.HasColorKey(0));
+        Assert.True(BlendMath.ColorKeyMatches(0, 0, 0, 0));      // (0,0,0) == key black
+        Assert.False(BlendMath.ColorKeyMatches(1, 0, 0, 0));    // near-black is NOT keyed (exact match)
+    }
+
+    [Fact]
+    public void ColorKey_MatchesRgb888Bytes()
+    {
+        long key = 0xFF00FF;                                    // magenta 0xRRGGBB
+        Assert.True(BlendMath.ColorKeyMatches(0xFF, 0x00, 0xFF, key));
+        Assert.False(BlendMath.ColorKeyMatches(0xFE, 0x00, 0xFF, key));
+    }
+
+    [Fact]
+    public void UnpackArgb_SplitsBytes()
+    {
+        Assert.Equal((0x80, 0x12, 0x34, 0x56), BlendMath.UnpackArgb(0x80123456));
+    }
+}