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/BuildingTests.cs

@@ -0,0 +1,44 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.App.Rendering.Wb;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering.Wb;
+
+public class BuildingTests
+{
+    [Fact]
+    public void Building_RequiredFields_PopulateCorrectly()
+    {
+        var b = new Building
+        {
+            BuildingId = 42,
+            EnvCellIds = new HashSet<uint> { 0xA9B40150u, 0xA9B40151u },
+            ExitPortalPolygons = new List<Vector3[]>
+            {
+                new[] { new Vector3(0, 0, 0), new Vector3(1, 0, 0), new Vector3(1, 1, 0) },
+            },
+        };
+
+        Assert.Equal(42u, b.BuildingId);
+        Assert.Equal(2, b.EnvCellIds.Count);
+        Assert.Contains(0xA9B40150u, b.EnvCellIds);
+        Assert.Single(b.ExitPortalPolygons);
+        Assert.Equal(3, b.ExitPortalPolygons[0].Length);
+    }
+
+    [Fact]
+    public void Building_OcclusionQueryState_DefaultsZero()
+    {
+        var b = new Building
+        {
+            BuildingId = 0,
+            EnvCellIds = new HashSet<uint>(),
+            ExitPortalPolygons = new List<Vector3[]>(),
+        };
+
+        Assert.Equal(0u, b.QueryId);
+        Assert.False(b.QueryStarted);
+        Assert.False(b.WasVisible);
+    }
+}