fix(streaming): commit EnvCell landblocks atomically (#214)

Carry one complete cell payload with each streaming completion and publish visibility, physics, and render state on the render thread. Remove the obsolete post-readiness login reload that triggered a duplicate dungeon build.

Co-Authored-By: Codex <noreply@openai.com>

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

@@ -0,0 +1,104 @@
+using System.Collections.Immutable;
+using System.Numerics;
+using AcDream.App.Rendering;
+using AcDream.App.Rendering.Wb;
+
+namespace AcDream.App.Tests.Rendering.Wb;
+
+public class EnvCellLandblockBuildTests
+{
+    private const uint LandblockId = 0x8C04FFFFu;
+
+    [Fact]
+    public void Constructor_SnapshotsInputCollections()
+    {
+        var cells = new List<LoadedCell> { new() { CellId = 0x8C040100u } };
+        var shells = new List<EnvCellShellPlacement> { Shell(0x8C040100u, 1) };
+
+        var build = new EnvCellLandblockBuild(LandblockId, cells, shells);
+        cells.Clear();
+        shells.Clear();
+
+        Assert.Single(build.VisibilityCells);
+        Assert.Single(build.Shells);
+    }
+
+    [Fact]
+    public void Constructor_RejectsCellFromAnotherLandblock()
+    {
+        var foreign = new LoadedCell { CellId = 0x00070100u };
+
+        Assert.Throws<ArgumentException>(() =>
+            new EnvCellLandblockBuild(LandblockId, new[] { foreign }, Array.Empty<EnvCellShellPlacement>()));
+    }
+
+    [Fact]
+    public void CreateCommittedSnapshot_ContainsEveryShellInOneCompleteReplacement()
+    {
+        var shells = Enumerable.Range(0, 1123)
+            .Select(index => Shell(0x8C040100u + (uint)index, (ulong)(index % 17 + 1)))
+            .ToArray();
+        var build = new EnvCellLandblockBuild(
+            LandblockId,
+            Array.Empty<LoadedCell>(),
+            shells);
+
+        var snapshot = EnvCellRenderer.CreateCommittedSnapshot(build);
+
+        Assert.Equal(1123, snapshot.Instances.Count);
+        Assert.Equal(1123, snapshot.EnvCellBounds.Count);
+        Assert.Equal(1123, snapshot.BuildingPartGroups.Values.Sum(group => group.Count));
+        Assert.True(snapshot.InstancesReady);
+        Assert.True(snapshot.MeshDataReady);
+        Assert.True(snapshot.GpuReady);
+    }
+
+    [Fact]
+    public void IndependentBuilds_NeverSharePendingStorage()
+    {
+        var complete = new EnvCellLandblockBuild(
+            LandblockId,
+            Array.Empty<LoadedCell>(),
+            Enumerable.Range(0, 1123)
+                .Select(index => Shell(0x8C040100u + (uint)index, 1)));
+        var second = new EnvCellLandblockBuild(
+            LandblockId,
+            Array.Empty<LoadedCell>(),
+            Enumerable.Range(0, 11)
+                .Select(index => Shell(0x8C040500u + (uint)index, 2)));
+
+        var completeSnapshot = EnvCellRenderer.CreateCommittedSnapshot(complete);
+        var secondSnapshot = EnvCellRenderer.CreateCommittedSnapshot(second);
+
+        Assert.Equal(1123, completeSnapshot.Instances.Count);
+        Assert.Equal(11, secondSnapshot.Instances.Count);
+        Assert.Equal(1123, completeSnapshot.Instances.Count);
+    }
+
+    [Fact]
+    public void Builder_CanPublishItsTransactionOnlyOnce()
+    {
+        var builder = new EnvCellLandblockBuildBuilder(LandblockId);
+
+        _ = builder.Build();
+
+        Assert.Throws<InvalidOperationException>(() => builder.Build());
+    }
+
+    private static EnvCellShellPlacement Shell(uint cellId, ulong geometryId)
+    {
+        var min = new Vector3(cellId & 0xFF, 0, 0);
+        var bounds = new WbBoundingBox(min, min + Vector3.One);
+        return new EnvCellShellPlacement(
+            cellId,
+            geometryId,
+            EnvironmentId: 1,
+            CellStructure: 0,
+            Surfaces: ImmutableArray.Create<ushort>(1),
+            WorldPosition: min,
+            Rotation: Quaternion.Identity,
+            Transform: Matrix4x4.CreateTranslation(min),
+            LocalBounds: new WbBoundingBox(Vector3.Zero, Vector3.One),
+            WorldBounds: bounds);
+    }
+}