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.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);