refactor(lighting): extract GlobalLightPacker (shared binding=4 layout) — A7 Fix D prep

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/AcDream.Core.Tests/Lighting/GlobalLightPackerTests.cs

@@ -0,0 +1,45 @@
+using System.Numerics;
+using AcDream.Core.Lighting;
+using Xunit;
+
+namespace AcDream.Core.Tests.Lighting;
+
+public class GlobalLightPackerTests
+{
+    [Fact]
+    public void Pack_WritesSixteenFloatsPerLight_InTheExpectedLayout()
+    {
+        var light = new LightSource
+        {
+            Kind          = LightKind.Point,
+            WorldPosition = new Vector3(10f, 20f, 30f),
+            WorldForward  = new Vector3(0f, 0f, 1f),
+            ColorLinear   = new Vector3(1.0f, 0.588f, 0.314f),
+            Intensity     = 100f,
+            Range         = 5.2f,
+            ConeAngle     = 0f,
+        };
+        float[] buffer = System.Array.Empty<float>();
+
+        int count = GlobalLightPacker.Pack(new[] { light }, ref buffer);
+
+        Assert.Equal(1, count);
+        Assert.True(buffer.Length >= 16);
+        Assert.Equal(10f, buffer[0]); Assert.Equal(20f, buffer[1]); Assert.Equal(30f, buffer[2]);
+        Assert.Equal((float)(int)LightKind.Point, buffer[3]);
+        Assert.Equal(0f, buffer[4]); Assert.Equal(0f, buffer[5]); Assert.Equal(1f, buffer[6]);
+        Assert.Equal(5.2f, buffer[7]);
+        Assert.Equal(1.0f, buffer[8]); Assert.Equal(0.588f, buffer[9]); Assert.Equal(0.314f, buffer[10]);
+        Assert.Equal(100f, buffer[11]);
+        Assert.Equal(0f, buffer[12]);
+    }
+
+    [Fact]
+    public void Pack_NullOrEmpty_ReturnsZero_AndBufferHasAtLeastOneSlot()
+    {
+        float[] buffer = System.Array.Empty<float>();
+        int count = GlobalLightPacker.Pack(null, ref buffer);
+        Assert.Equal(0, count);
+        Assert.True(buffer.Length >= GlobalLightPacker.FloatsPerLight);
+    }
+}