fix(core): UCG Stage 1 — final-review polish (VisibleCells null-guard, Neighbor doc)

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

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

@@ -49,5 +49,10 @@ public sealed class CellGraph
         return LandCell.Synthesize(id, t.Terrain, t.Origin, idx / 8, idx % 8);
     }
 
+    /// <summary>
+    /// Resolve the cell on the far side of a portal — retail CCellPortal::GetOtherCell =
+    /// GetVisible(other_cell_id). <paramref name="cell"/> documents the traversal source and is
+    /// reserved for the OtherCellPtr neighbor cache (Stage 3); the lookup keys only on the portal.
+    /// </summary>
     public ObjCell? Neighbor(ObjCell cell, in CellPortal portal) => GetVisible(portal.OtherCellId);
 }