feat(core): UCG Stage 1 — EnvCell.FromDat derivation (mirrors BuildLoadedCell)

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

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

@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.Core.World.Cells;
+using DatReaderWriter.Enums;
+using DatReaderWriter.Types;
+using Xunit;
+using DatEnvCell = DatReaderWriter.DBObjs.EnvCell;
+using DatCellPortal = DatReaderWriter.Types.CellPortal;
+
+namespace AcDream.Core.Tests.World.Cells;
+
+public class EnvCellFromDatTests
+{
+    [Fact]
+    public void FromDat_DerivesSeenOutside_OtherPortalId_PrefixedStab_AndBoundsFallback()
+    {
+        var cellStruct = new CellStruct
+        {
+            VertexArray = new VertexArray { Vertices = new Dictionary<ushort, SWVertex>() }, // empty -> bounds fallback
+            Polygons = new Dictionary<ushort, Polygon>(),
+        }; // CellBSP defaults to null → ContainmentBsp will be null
+        var dat = new DatEnvCell
+        {
+            Flags = EnvCellFlags.SeenOutside,
+            CellPortals = new List<DatCellPortal>
+            {
+                new() { OtherCellId = 0x0105, PolygonId = 0, OtherPortalId = 7, Flags = (PortalFlags)0 },
+            },
+            VisibleCells = new List<ushort> { 0x0105, 0x0106 },
+        };
+
+        var env = EnvCell.FromDat(0xA9B40104u, dat, cellStruct, Matrix4x4.Identity);
+
+        Assert.True(env.SeenOutside);
+        Assert.Single(env.Portals);
+        Assert.Equal(0x0105u, env.Portals[0].OtherCellId);
+        Assert.Equal((ushort)7, env.Portals[0].OtherPortalId);
+        Assert.Equal(new[] { 0xA9B40105u, 0xA9B40106u }, env.StabList);
+        Assert.Equal(Vector3.Zero, env.LocalBoundsMin);
+        Assert.Equal(Vector3.Zero, env.LocalBoundsMax);
+        Assert.Null(env.ContainmentBsp);
+    }
+}