feat(core): UCG Stage 1 — ObjCell base + CellPortal

Introduces AcDream.Core.World.Cells namespace with the two foundational
types for the Unified Cell Graph. CellPortal is a readonly struct
unifying the three legacy portal representations; ObjCell is the abstract
base for all traversable cells with the retail id-magnitude IsEnv
discriminator (CObjCell::GetVisible, pseudo_c:308215). Zero consumers;
zero behavior change. 5/5 tests green.

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

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

@@ -0,0 +1,43 @@
+using System.Collections.Generic;
+using System.Numerics;
+
+namespace AcDream.Core.World.Cells;
+
+/// <summary>
+/// Base for every cell the player can stand in. Retail anchor: CObjCell
+/// (acclient.h:30915). The id magnitude is the type discriminator
+/// (<see cref="IsEnv"/>): low-16 &gt;= 0x100 =&gt; indoor <see cref="EnvCell"/>,
+/// else outdoor <see cref="LandCell"/>.
+/// </summary>
+public abstract class ObjCell
+{
+    public uint Id { get; }
+    public Matrix4x4 WorldTransform { get; }
+    public Matrix4x4 InverseWorldTransform { get; }
+    public Vector3 LocalBoundsMin { get; }
+    public Vector3 LocalBoundsMax { get; }
+    public IReadOnlyList<CellPortal> Portals { get; }
+    public IReadOnlyList<uint> StabList { get; }
+    public bool SeenOutside { get; }
+
+    /// <summary>Retail magnitude dispatch (CObjCell::GetVisible, pseudo_c:308215).</summary>
+    public bool IsEnv => (Id & 0xFFFFu) >= 0x100u;
+
+    protected ObjCell(uint id, Matrix4x4 worldTransform, Matrix4x4 inverseWorldTransform,
+                      Vector3 localBoundsMin, Vector3 localBoundsMax,
+                      IReadOnlyList<CellPortal> portals, IReadOnlyList<uint> stabList,
+                      bool seenOutside)
+    {
+        Id = id;
+        WorldTransform = worldTransform;
+        InverseWorldTransform = inverseWorldTransform;
+        LocalBoundsMin = localBoundsMin;
+        LocalBoundsMax = localBoundsMax;
+        Portals = portals;
+        StabList = stabList;
+        SeenOutside = seenOutside;
+    }
+
+    /// <summary>Retail CObjCell::point_in_cell (vtable +0x84). Is a world point inside this cell?</summary>
+    public abstract bool PointInCell(Vector3 worldPoint);
+}