feat(core): UCG Stage 1 — LandCell synthesized from TerrainSurface

Outdoor terrain cell (retail CLandCell) synthesized on demand from a
landblock's TerrainSurface. Factory Synthesize() samples four quad
corners to establish Z bounds; PointInCell() tests the 24 m XY quad
in world-local space. BuildingCellId stub is null (Stage 2).
2/2 tests RED→GREEN.

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

tests/AcDream.Core.Tests/World/Cells/LandCellTests.cs

@@ -0,0 +1,36 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+using AcDream.Core.World.Cells;
+using Xunit;
+
+namespace AcDream.Core.Tests.World.Cells;
+
+public class LandCellTests
+{
+    private static TerrainSurface FlatTerrain()
+        => new TerrainSurface(new byte[81], new float[256], landblockX: 0, landblockY: 0);
+
+    [Fact]
+    public void Synthesize_SetsCellIndicesAndQuadBounds()
+    {
+        var origin = new Vector3(1000f, 2000f, 0f);
+        var cell = LandCell.Synthesize(0xA9B40014u, FlatTerrain(), origin, cx: 2, cy: 3);
+        Assert.Equal(2, cell.Cx);
+        Assert.Equal(3, cell.Cy);
+        Assert.Equal(2 * 24f, cell.LocalBoundsMin.X);
+        Assert.Equal(3 * 24f, cell.LocalBoundsMin.Y);
+        Assert.Equal(3 * 24f, cell.LocalBoundsMax.X);
+        Assert.Equal(4 * 24f, cell.LocalBoundsMax.Y);
+        Assert.False(cell.IsEnv);
+    }
+
+    [Fact]
+    public void PointInCell_TestsWorldXyAgainstThe24mQuad()
+    {
+        var origin = new Vector3(1000f, 2000f, 0f);
+        var cell = LandCell.Synthesize(0xA9B40014u, FlatTerrain(), origin, cx: 2, cy: 3);
+        Assert.True(cell.PointInCell(new Vector3(1060f, 2080f, 12.3f)));
+        Assert.False(cell.PointInCell(new Vector3(1000f, 2080f, 0f)));
+        Assert.False(cell.PointInCell(new Vector3(1060f, 2100f, 0f)));
+    }
+}