feat(render): Phase A8 RR3 — Building + BuildingRegistry + BuildingLoader

New per-landblock data model for WB-style per-building cell scoping:

  Building            — BuildingId, EnvCellIds, ExitPortalPolygons,
                        occlusion-query state (Step 5 lifecycle)
  BuildingRegistry    — two-way indexed (by cellId + by buildingId);
                        single source of truth per landblock
  BuildingLoader      — static factory from LandBlockInfo.Buildings;
                        walks interior portals to expand cell sets;
                        collects exit portal polygons in world space

10 new unit tests cover data invariants + registry indexing + loader
mapping per the algorithm resolved in RR2 findings.

LoadedCell.BuildingId stamping wired in RR4. Render-time consumption
arrives in RR7 (Steps 1-4) + RR9 (Step 5) + RR11 (RenderOutsideIn).

Design: docs/superpowers/specs/2026-05-26-phase-a8-wb-full-port-design.md
Spike: docs/research/2026-05-26-a8-buildings-data-shape.md

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

tests/AcDream.App.Tests/Rendering/Wb/BuildingLoaderTests.cs

@@ -0,0 +1,88 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.App.Rendering.Wb;
+using DatReaderWriter.DBObjs;
+using DatReaderWriter.Types;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering.Wb;
+
+public class BuildingLoaderTests
+{
+    // Helper: build a minimal LandBlockInfo with one BuildingInfo per supplied tuple.
+    // OtherCellId values are 16-bit cell-local ids (low word of full cell id).
+    private static LandBlockInfo MakeInfo(params (uint modelId, uint[] portalOtherCellIds)[] buildings)
+    {
+        var bls = new List<BuildingInfo>();
+        foreach (var (modelId, portals) in buildings)
+        {
+            var portalList = new List<BuildingPortal>();
+            foreach (var ocid in portals)
+            {
+                portalList.Add(new BuildingPortal
+                {
+                    OtherCellId  = (ushort)(ocid & 0xFFFFu),
+                    Flags        = 0,
+                    OtherPortalId = 0,
+                    StabList     = new List<ushort>(),
+                });
+            }
+            bls.Add(new BuildingInfo
+            {
+                ModelId  = modelId,
+                Frame    = new Frame { Origin = Vector3.Zero, Orientation = Quaternion.Identity },
+                Portals  = portalList,
+            });
+        }
+        return new LandBlockInfo
+        {
+            Objects   = new List<Stab>(),
+            Buildings = bls,
+        };
+    }
+
+    [Fact]
+    public void Empty_NoBuildings_EmptyRegistry()
+    {
+        var info = new LandBlockInfo { Objects = new List<Stab>(), Buildings = new List<BuildingInfo>() };
+        var reg = BuildingLoader.Build(info, landblockId: 0xA9B40000u, cellsByCellId: new Dictionary<uint, AcDream.App.Rendering.LoadedCell>());
+        Assert.Equal(0, reg.Count);
+    }
+
+    [Fact]
+    public void OneBuilding_OnePortal_MapsToOneCell()
+    {
+        // Building points to cell 0x0150 in landblock 0xA9B40000 → full cell id 0xA9B40150
+        var info = MakeInfo((modelId: 0x02000123u, portalOtherCellIds: new[] { 0x0150u }));
+        // Pass an empty cell dict — loader seeds from BuildingInfo.Portals only
+        var reg = BuildingLoader.Build(info, landblockId: 0xA9B40000u, cellsByCellId: new Dictionary<uint, AcDream.App.Rendering.LoadedCell>());
+        Assert.Equal(1, reg.Count);
+        var building = System.Linq.Enumerable.First(reg.All());
+        Assert.Contains(0xA9B40150u, building.EnvCellIds);
+    }
+
+    [Fact]
+    public void OneBuilding_MultiplePortals_MapsToMultipleCells()
+    {
+        var info = MakeInfo((0x02000123u, new[] { 0x0150u, 0x0151u, 0x0152u }));
+        var reg = BuildingLoader.Build(info, 0xA9B40000u, new Dictionary<uint, AcDream.App.Rendering.LoadedCell>());
+        var building = System.Linq.Enumerable.First(reg.All());
+        Assert.Equal(3, building.EnvCellIds.Count);
+        Assert.Contains(0xA9B40150u, building.EnvCellIds);
+        Assert.Contains(0xA9B40151u, building.EnvCellIds);
+        Assert.Contains(0xA9B40152u, building.EnvCellIds);
+    }
+
+    [Fact]
+    public void TwoBuildings_AllocateSequentialIds()
+    {
+        var info = MakeInfo(
+            (0x02000001u, new[] { 0x0150u }),
+            (0x02000002u, new[] { 0x0160u }));
+        var reg = BuildingLoader.Build(info, 0xA9B40000u, new Dictionary<uint, AcDream.App.Rendering.LoadedCell>());
+        Assert.Equal(2, reg.Count);
+        var ids = new SortedSet<uint>();
+        foreach (var b in reg.All()) ids.Add(b.BuildingId);
+        Assert.Equal(new SortedSet<uint> { 1, 2 }, ids);  // sequential 1, 2
+    }
+}