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));
+    }
+}

engine/Age.Engine/Model/BlendMath.cs

@@ -0,0 +1,22 @@
+namespace Age.Engine.Model;
+
+/// <summary>Pure blend helpers shared by the engine resolution and the host blit. Colorkey semantics come
+/// from the native loader (docs/engine-re.md §"Blend & transparency"): op arg 3 &lt; 0 = no key; otherwise
+/// the operand is 0xRRGGBB and matching (R,G,B) texels are transparent (operand 0 = key black).</summary>
+public static class BlendMath
+{
+    /// <summary>A colorkey operand &gt;= 0 is an active key; a negative operand means "no colorkey".</summary>
+    public static bool HasColorKey(long colorKey) => colorKey >= 0;
+
+    /// <summary>True when (r,g,b) exactly equals the key's 0xRRGGBB bytes. Always false when there is no key.</summary>
+    public static bool ColorKeyMatches(byte r, byte g, byte b, long colorKey)
+    {
+        if (!HasColorKey(colorKey)) return false;
+        return r == ((colorKey >> 16) & 0xff) && g == ((colorKey >> 8) & 0xff) && b == (colorKey & 0xff);
+    }
+
+    /// <summary>Split a 0xAARRGGBB packed color (see GfxState.PackColor) into its byte channels.</summary>
+    public static (int A, int R, int G, int B) UnpackArgb(long packed)
+        => ((int)((packed >> 24) & 0xff), (int)((packed >> 16) & 0xff),
+            (int)((packed >> 8) & 0xff), (int)(packed & 0xff));
+}