feat(gfx): GfxState host-agnostic command-buffer model

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

engine/Age.Engine.Tests/GfxStateTests.cs

@@ -0,0 +1,41 @@
+using Age.Engine.Model;
+using Xunit;
+
+public class GfxStateTests
+{
+    [Fact]
+    public void DistinctHandlesGetDistinctSlots()
+    {
+        var g = new GfxState();
+        int s1 = g.GetOrCreate(0x1000).Slot;
+        int s2 = g.GetOrCreate(0x2000).Slot;
+        Assert.NotEqual(s1, s2);
+        Assert.Equal(s1, g.QuerySlot(0x1000));   // stable
+        Assert.Equal(-1, g.QuerySlot(0x9999));    // unknown -> -1 (matches native 0xffffffff)
+    }
+
+    [Fact]
+    public void VectorsRoundTripPerObject()
+    {
+        var g = new GfxState();
+        g.GetOrCreate(0x1000).V18 = (10, 20, 30);
+        g.GetOrCreate(0x1000).V24 = (40, 50, 60);
+        Assert.Equal((10L, 20L, 30L), g.TryGet(0x1000)!.V18);
+        Assert.Equal((40L, 50L, 60L), g.TryGet(0x1000)!.V24);
+        Assert.Null(g.TryGet(0x2000));            // untouched handle absent
+    }
+
+    [Fact]
+    public void ReleaseFreesTheSlotForReuse()
+    {
+        var g = new GfxState();
+        int s1 = g.GetOrCreate(0x1000).Slot;
+        g.Release(0x1000);
+        Assert.Equal(-1, g.QuerySlot(0x1000));
+        Assert.Equal(s1, g.GetOrCreate(0x2000).Slot);  // freed slot reused
+    }
+
+    [Fact]
+    public void PackColorPacksArgb()
+        => Assert.Equal(0x80_112233L, GfxState.PackColor(0x80, 0x112233));
+}