feat(core): UCG Stage 1 — CellGraph resolver + registry + inert CurrCell

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

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

@@ -0,0 +1,72 @@
+using System.Numerics;
+using AcDream.Core.Physics;
+using AcDream.Core.World.Cells;
+using Xunit;
+
+namespace AcDream.Core.Tests.World.Cells;
+
+public class CellGraphTests
+{
+    private static TerrainSurface FlatTerrain() => new TerrainSurface(new byte[81], new float[256]);
+
+    private static EnvCell Env(uint id) => new EnvCell(id, Matrix4x4.Identity, Matrix4x4.Identity,
+        Vector3.Zero, new Vector3(10,10,10), System.Array.Empty<CellPortal>(),
+        System.Array.Empty<uint>(), false, null);
+
+    [Fact]
+    public void GetVisible_ZeroId_ReturnsNull()
+        => Assert.Null(new CellGraph().GetVisible(0u));
+
+    [Fact]
+    public void GetVisible_EnvId_ReturnsAddedEnvCell()
+    {
+        var g = new CellGraph();
+        var env = Env(0xA9B40174u);
+        g.Add(env);
+        Assert.Same(env, g.GetVisible(0xA9B40174u));
+    }
+
+    [Fact]
+    public void GetVisible_UnknownEnvId_ReturnsNull()
+        => Assert.Null(new CellGraph().GetVisible(0xA9B40174u));
+
+    [Fact]
+    public void GetVisible_LandId_SynthesizesFromRegisteredTerrain()
+    {
+        var g = new CellGraph();
+        g.RegisterTerrain(0xA9B40000u, FlatTerrain(), new Vector3(1000,2000,0));
+        var cell = g.GetVisible(0xA9B40014u);
+        var land = Assert.IsType<LandCell>(cell);
+        Assert.Equal(2, land.Cx);
+        Assert.Equal(3, land.Cy);
+    }
+
+    [Fact]
+    public void GetVisible_LandId_NoTerrain_ReturnsNull()
+        => Assert.Null(new CellGraph().GetVisible(0xA9B40014u));
+
+    [Fact]
+    public void RemoveLandblock_EvictsEnvAndTerrain()
+    {
+        var g = new CellGraph();
+        g.Add(Env(0xA9B40174u));
+        g.RegisterTerrain(0xA9B40000u, FlatTerrain(), Vector3.Zero);
+        g.RemoveLandblock(0xA9B40000u);
+        Assert.Null(g.GetVisible(0xA9B40174u));
+        Assert.Null(g.GetVisible(0xA9B40014u));
+    }
+
+    [Fact]
+    public void Neighbor_ResolvesPortalOtherCellId()
+    {
+        var g = new CellGraph();
+        var target = Env(0xA9B40175u);
+        g.Add(target);
+        var portal = new CellPortal(0xA9B40175u, 0, 0, 0);
+        Assert.Same(target, g.Neighbor(Env(0xA9B40174u), portal));
+    }
+
+    [Fact]
+    public void CurrCell_IsNull_InStage1()
+        => Assert.Null(new CellGraph().CurrCell);
+}