feat(core): UCG Stage 1 — EnvCell + PointInCell (AABB/BSP)

Adds `EnvCell` (sealed, extends `ObjCell`) with a primitive constructor
and `PointInCell` that uses the cell-containment BSP when present, else
falls back to an AABB test. Retail anchor: CEnvCell (acclient.h:32072).
BSP branch delegates to `BSPQuery.PointInsideCellBsp` (BSPQuery.cs:1034);
the AABB branch is the genuinely new logic. No `FromDat` factory — that is
a separate later task. Consumed by nobody yet (Stage 1 scaffold).

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

src/AcDream.Core/World/Cells/EnvCell.cs

@@ -0,0 +1,33 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.Physics;          // BSPQuery
+using DatReaderWriter.Types;          // CellBSPTree
+
+namespace AcDream.Core.World.Cells;
+
+/// <summary>Indoor room cell. Retail anchor: CEnvCell (acclient.h:32072).</summary>
+public sealed class EnvCell : ObjCell
+{
+    /// <summary>Cell-containment BSP (retail CellStruct.CellBSP). Null =&gt; AABB fallback.</summary>
+    public CellBSPTree? ContainmentBsp { get; }
+
+    public EnvCell(uint id, Matrix4x4 worldTransform, Matrix4x4 inverseWorldTransform,
+                   Vector3 localBoundsMin, Vector3 localBoundsMax,
+                   IReadOnlyList<CellPortal> portals, IReadOnlyList<uint> stabList,
+                   bool seenOutside, CellBSPTree? containmentBsp)
+        : base(id, worldTransform, inverseWorldTransform, localBoundsMin, localBoundsMax,
+               portals, stabList, seenOutside)
+    {
+        ContainmentBsp = containmentBsp;
+    }
+
+    public override bool PointInCell(Vector3 worldPoint)
+    {
+        var local = Vector3.Transform(worldPoint, InverseWorldTransform);
+        if (ContainmentBsp?.Root is not null)
+            return BSPQuery.PointInsideCellBsp(ContainmentBsp.Root, local);   // BSPQuery.cs:1034
+        return local.X >= LocalBoundsMin.X && local.X <= LocalBoundsMax.X
+            && local.Y >= LocalBoundsMin.Y && local.Y <= LocalBoundsMax.Y
+            && local.Z >= LocalBoundsMin.Z && local.Z <= LocalBoundsMax.Z;
+    }
+}

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

@@ -0,0 +1,33 @@
+using System.Numerics;
+using AcDream.Core.World.Cells;
+using Xunit;
+
+namespace AcDream.Core.Tests.World.Cells;
+
+public class EnvCellTests
+{
+    private static EnvCell Make(Vector3 min, Vector3 max, Matrix4x4? transform = null)
+    {
+        var t = transform ?? Matrix4x4.Identity;
+        Matrix4x4.Invert(t, out var inv);
+        return new EnvCell(0xA9B40174u, t, inv, min, max,
+            System.Array.Empty<CellPortal>(), System.Array.Empty<uint>(),
+            seenOutside: false, containmentBsp: null);
+    }
+
+    [Fact]
+    public void PointInCell_NullBsp_Aabb_InsideIsTrue()
+        => Assert.True(Make(new Vector3(0,0,0), new Vector3(10,10,10)).PointInCell(new Vector3(5,5,5)));
+
+    [Fact]
+    public void PointInCell_NullBsp_Aabb_OutsideIsFalse()
+        => Assert.False(Make(new Vector3(0,0,0), new Vector3(10,10,10)).PointInCell(new Vector3(20,5,5)));
+
+    [Fact]
+    public void PointInCell_TransformsWorldToLocalBeforeTesting()
+    {
+        var c = Make(new Vector3(0,0,0), new Vector3(10,10,10), Matrix4x4.CreateTranslation(100,0,0));
+        Assert.True(c.PointInCell(new Vector3(105,5,5)));
+        Assert.False(c.PointInCell(new Vector3(5,5,5)));
+    }
+}