feat(render): Phase A8 Wave 1 — WB scaffolding extraction + stencil low-level method

Five tasks shipped together (interdependent at build time):

Task 1: WbRenderPass enum — verbatim port of WB RenderPass.cs:1-22
Task 2: WbFrustum + WbBoundingBox + FrustumTestResult — verbatim port
  of WB Frustum.cs (98 LOC) with namespace + BoundingBox-type adaptations.
  +7 unit tests.
Task 3: EnvCellSceneryInstance + EnvCellLandblock — verbatim port of WB
  SceneryInstance.cs:1-161, renamed scope-narrow. Dropped editor-only
  fields (DisqualificationReason, ParticleEmitters, IsQueuedForUpload,
  InstanceBufferOffset, InstanceCount, MdiCommands, IsTransformOnlyUpdate)
  + InstanceId narrowed uint (we don't use ObjectId's editor methods).
  +5 unit tests.
Task 4: EnvCellVisibilitySnapshot — direct port of WB VisibilitySnapshot
  narrowed to BatchedByCell + VisibleLandblocks only.
Task 7: IndoorCellStencilPipeline.RenderBuildingStencilMask — new
  low-level WB-faithful entry mirroring PortalRenderManager:471-484.
  No surrounding GL state setup (caller's responsibility). Probe fields
  LastStencilVertexCount / LastStencilWasFarPunch / LastStencilBuildingId
  for the [stencil] probe emitter in Task 9.

Build green, 18 tests pass (7 new Frustum + 5 new SceneryInstance + 6
existing stencil pipeline). Ready for Wave 2 (EnvCellRenderer port).

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

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

@@ -0,0 +1,78 @@
+using System.Collections.Generic;
+using System.Numerics;
+using AcDream.App.Rendering.Wb;
+using Xunit;
+
+namespace AcDream.App.Tests.Rendering.Wb;
+
+public class EnvCellSceneryInstanceTests
+{
+    [Fact]
+    public void Instance_DefaultConstruct_HasZeroFields()
+    {
+        var s = new EnvCellSceneryInstance();
+        Assert.Equal(0UL, s.ObjectId);
+        Assert.False(s.IsBuilding);
+        Assert.False(s.IsSetup);
+        Assert.False(s.IsEntryCell);
+    }
+
+    [Fact]
+    public void Instance_AssignFields_RoundTrip()
+    {
+        var t = Matrix4x4.CreateTranslation(1, 2, 3);
+        var s = new EnvCellSceneryInstance
+        {
+            ObjectId = 0x01000123,
+            IsBuilding = true,
+            IsSetup = false,
+            IsEntryCell = true,
+            WorldPosition = new Vector3(1, 2, 3),
+            Rotation = Quaternion.Identity,
+            Scale = Vector3.One,
+            Transform = t,
+        };
+        Assert.Equal(0x01000123UL, s.ObjectId);
+        Assert.True(s.IsBuilding);
+        Assert.True(s.IsEntryCell);
+        Assert.Equal(new Vector3(1, 2, 3), s.WorldPosition);
+        Assert.Equal(t, s.Transform);
+    }
+
+    [Fact]
+    public void Landblock_Construct_StartsEmpty()
+    {
+        var lb = new EnvCellLandblock { GridX = 0xA9, GridY = 0xB4 };
+        Assert.Empty(lb.StaticPartGroups);
+        Assert.Empty(lb.BuildingPartGroups);
+        Assert.Empty(lb.Instances);
+        Assert.Empty(lb.EnvCellBounds);
+        Assert.False(lb.InstancesReady);
+        Assert.False(lb.GpuReady);
+        Assert.False(lb.MeshDataReady);
+    }
+
+    [Fact]
+    public void Landblock_AddInstanceToBuildingPartGroups_PreservesOrder()
+    {
+        var lb = new EnvCellLandblock();
+        if (!lb.BuildingPartGroups.TryGetValue(0x01000001UL, out var list))
+        {
+            list = new List<InstanceData>();
+            lb.BuildingPartGroups[0x01000001UL] = list;
+        }
+        list.Add(default);
+        list.Add(default);
+        Assert.Single(lb.BuildingPartGroups);
+        Assert.Equal(2, lb.BuildingPartGroups[0x01000001UL].Count);
+    }
+
+    [Fact]
+    public void Landblock_PendingInstancesNullByDefault()
+    {
+        var lb = new EnvCellLandblock();
+        Assert.Null(lb.PendingInstances);
+        Assert.Null(lb.PendingEnvCellBounds);
+        Assert.Null(lb.PendingSeenOutsideCells);
+    }
+}