feat: resolve per-object alpha/tint/blend into RenderObject (colorkey retained)

0x202/0x203 now route through GfxState.SetObjectColor (sets HasColor);
SnapshotVisibleObjects resolves Alpha/Tint/BlendKind. Drops the stale
'alpha deferred' trace stub — alpha/tint is now consumed by the compositor.

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

engine/Age.Engine.Tests/RenderObjectBlendTests.cs

@@ -0,0 +1,45 @@
+using System.Linq;
+using Age.Engine.Model;
+using Xunit;
+
+public class RenderObjectBlendTests
+{
+    // Make a visible textured object bound to a slot that has a surface, so it appears in the snapshot.
+    private static GfxState WithVisibleObject(long handle, long resId, long colorKey)
+    {
+        var g = new GfxState();
+        g.SetSurface(1, resId, colorKey);
+        g.BindDraw(handle, 1, 0, 0, 10, 10, 0, 0);   // draw-texture: slot 1, 10x10 at (0,0), visible
+        return g;
+    }
+
+    [Fact]
+    public void ObjectWithoutColor_ResolvesOpaqueWhiteTint()
+    {
+        var g = WithVisibleObject(0x100, resId: 5, colorKey: -1);
+        var ro = g.SnapshotVisibleObjects().Single();
+        Assert.Equal(255, ro.Alpha);
+        Assert.Equal(0xFFFFFF, ro.Tint);
+        Assert.Equal(BlendKind.Opaque, ro.Blend);
+    }
+
+    [Fact]
+    public void DrawColor_0x203_SetsAlphaTintAndBlend()
+    {
+        var g = WithVisibleObject(0x100, resId: 5, colorKey: -1);
+        // emulate op 0x203: pack (alpha=0x80, color=0x102030) and mark HasColor
+        g.SetObjectColor(0x100, GfxState.PackColor(0x80, 0x102030));
+        var ro = g.SnapshotVisibleObjects().Single();
+        Assert.Equal(0x80, ro.Alpha);
+        Assert.Equal(0x102030, ro.Tint);
+        Assert.Equal(BlendKind.Alpha, ro.Blend);
+    }
+
+    [Fact]
+    public void ColorKey_IsCarriedThrough()
+    {
+        var g = WithVisibleObject(0x100, resId: 5, colorKey: 0x000000);   // key black
+        var ro = g.SnapshotVisibleObjects().Single();
+        Assert.Equal(0x000000, ro.ColorKey);
+    }
+}