feat(world): Phase A8 R1 — tag WorldEntity.IsBuildingShell at LandblockLoader

Adds a bool flag at the WorldEntity data layer set by LandblockLoader from
the source dat array: LandBlockInfo.Buildings → true (cottage walls, inn
walls, smithy walls); LandBlockInfo.Objects → false (trees, lampposts,
rocks, hitching posts).

Retail anchor: CLandBlock::init_buildings reads a separate BuildInfo**
array from objects (acclient.h:31893 num_buildings / buildings field;
acclient_2013_pseudo_c.txt:313854 init_buildings entry). WorldBuilder
preserves the same distinction via SceneryInstance.IsBuilding
(StaticObjectRenderManager.cs:334). Today acdream's loader reads both
arrays into the same WorldEntity pool with no tag, destroying the
distinction (the comment at GameWindow.cs:5175 already acknowledges this
gap for scenery suppression). This commit closes the gap.

Render-time consumption arrives in R2 (EntitySet partition refactor).
Two new LandblockLoader tests lock the tagging behavior.

Co-Authored-By: Claude Opus 4.7 (1M context) <noreply@anthropic.com>

src/AcDream.App/Rendering/GameWindow.cs

@@ -5133,6 +5133,7 @@ public sealed class GameWindow : IDisposable
                 Position = e.Position + worldOffset,
                 Rotation = e.Rotation,
                 MeshRefs = meshRefs,
+                IsBuildingShell = e.IsBuildingShell,  // Phase A8: preserve dat-level tag
             };
             hydrated.Add(entity);
         }

src/AcDream.Core/World/LandblockLoader.cs

@@ -82,6 +82,7 @@ public static class LandblockLoader
                 Position = building.Frame.Origin,
                 Rotation = building.Frame.Orientation,
                 MeshRefs = Array.Empty<MeshRef>(),
+                IsBuildingShell = true,  // Phase A8: tag at source array boundary
             };
             buildingEntity.RefreshAabb(); // A.5 T18: populate cached AABB at construction
             result.Add(buildingEntity);

src/AcDream.Core/World/WorldEntity.cs

@@ -44,6 +44,25 @@ public sealed class WorldEntity
     /// </summary>
     public uint? ParentCellId { get; init; }
 
+    /// <summary>
+    /// True when this entity originates from <c>LandBlockInfo.Buildings[]</c>
+    /// (the dat array that carries building shells: cottage walls, smithy walls,
+    /// inn walls — every solid building enclosure). False for entities from
+    /// <c>LandBlockInfo.Objects[]</c> (rocks, fences, lampposts, tree clusters —
+    /// outdoor scenery placeholders). The two arrays are conflated through
+    /// hydration today but the dat itself carries the distinction; retail
+    /// (<c>CLandBlock::init_buildings</c>) and WorldBuilder
+    /// (<c>SceneryInstance.IsBuilding</c>) both preserve it.
+    ///
+    /// <para>
+    /// Read at draw time by <c>WbDrawDispatcher</c>'s <c>IndoorPass</c>
+    /// partition so building shells render unconditionally when the camera
+    /// is inside their building (they ARE the indoor walls), not stencil-gated
+    /// as outdoor scenery would be.
+    /// </para>
+    /// </summary>
+    public bool IsBuildingShell { get; init; }
+
     /// <summary>
     /// Uniform scale applied to this entity's mesh by the scenery pipeline.
     /// For scenery objects this is spawn.Scale (typically 0.8–1.3). For stabs