feat(render): Phase U.4c — LoadedCell carries stab_list PVS + seen_outside

VisibleCells (full ids) + SeenOutside, populated at the EnvCell-build site from
envCell.VisibleCells + envCell.Flags. Mirrors retail CEnvCell.stab_list /
seen_outside (acclient.h ~30925). Data already in-process; render path no longer
drops it. Consumed by the builder in U.4c-3.

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

src/AcDream.App/Rendering/CellVisibility.cs

@@ -83,6 +83,25 @@ public sealed class LoadedCell
     /// and route IndoorPass cell scoping.</para>
     /// </summary>
     public uint? BuildingId { get; internal set; }
+
+    /// <summary>
+    /// Phase U.4c: the stab_list PVS as full (landblock-prefixed) cell ids — retail
+    /// CEnvCell.stab_list (acclient.h ~30925), the stable set of cells potentially
+    /// visible from this cell, precomputed by the AC content tools. Refreshed only at
+    /// hydration (= retail's per-cell-entry grab_visible_cells, decomp:311878).
+    /// PortalVisibilityBuilder grounds set membership in it so a brittle per-frame
+    /// portal-side test can't drop a potentially-visible cell from the visible set.
+    /// Empty when the dat carried no stab list (degenerate / old cell).
+    /// </summary>
+    public IReadOnlyList<uint> VisibleCells = System.Array.Empty<uint>();
+
+    /// <summary>
+    /// Phase U.4c: retail CEnvCell.seen_outside (acclient.h ~30925) — this cell sees
+    /// the exterior (an exit portal is reachable from it). Retail gates the landscape
+    /// data + draw decision on the camera cell's value (RenderNormalMode decomp:92649,
+    /// grab_visible_cells decomp:311878). The stable anchor for the terrain-draw test.
+    /// </summary>
+    public bool SeenOutside;
 }
 
 /// <summary>