feat(A.5 T14): GpuWorldState RemoveEntitiesFromLandblock + AddEntitiesToExisting

Two new methods on GpuWorldState, used by two-tier streaming (T13):

- RemoveEntitiesFromLandblock(id): drop all entities from an LB while
  keeping the terrain. Used for Near->Far demote (player walks past the
  inner ring; LB stays loaded but entities leave).
- AddEntitiesToExistingLandblock(id, entities): merge new entities into
  an already-loaded LB record. Used for Far->Near promote (terrain is
  already on the GPU; just streaming the entity layer in).

Falls back to the pending bucket if the LB hasn't loaded yet.

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

tests/AcDream.Core.Tests/Streaming/GpuWorldStateTwoTierTests.cs

@@ -0,0 +1,76 @@
+using System.Linq;
+using AcDream.App.Streaming;
+using AcDream.Core.World;
+using DatReaderWriter.DBObjs;
+using Xunit;
+
+namespace AcDream.Core.Tests.Streaming;
+
+public class GpuWorldStateTwoTierTests
+{
+    private static LoadedLandblock MakeStubLandblock(uint canonicalId, params WorldEntity[] entities)
+        => new(canonicalId, new LandBlock(), entities);
+
+    private static WorldEntity MakeStubEntity(uint id)
+        => new()
+        {
+            Id = id,
+            SourceGfxObjOrSetupId = 0x01000001u,
+            Position = System.Numerics.Vector3.Zero,
+            Rotation = System.Numerics.Quaternion.Identity,
+            MeshRefs = System.Array.Empty<MeshRef>(),
+        };
+
+    [Fact]
+    public void RemoveEntitiesFromLandblock_KeepsLandblockButDropsEntities()
+    {
+        var state = new GpuWorldState();
+        var lb = MakeStubLandblock(0xAAAAFFFFu,
+            MakeStubEntity(1),
+            MakeStubEntity(2));
+        state.AddLandblock(lb);
+        Assert.Equal(2, state.Entities.Count);
+
+        state.RemoveEntitiesFromLandblock(0xAAAAFFFFu);
+
+        Assert.Empty(state.Entities);
+        Assert.True(state.IsLoaded(0xAAAAFFFFu));   // landblock still resident
+    }
+
+    [Fact]
+    public void AddEntitiesToExistingLandblock_MergesIntoExistingRecord()
+    {
+        var state = new GpuWorldState();
+        var lb = MakeStubLandblock(0xAAAAFFFFu, MakeStubEntity(1));
+        state.AddLandblock(lb);
+
+        state.AddEntitiesToExistingLandblock(0xAAAAFFFFu, new[]
+        {
+            MakeStubEntity(2),
+            MakeStubEntity(3),
+        });
+
+        Assert.Equal(3, state.Entities.Count);
+    }
+
+    [Fact]
+    public void AddEntitiesToExistingLandblock_LandblockNotYetLoaded_ParksInPending()
+    {
+        var state = new GpuWorldState();
+
+        // Landblock not loaded yet.
+        state.AddEntitiesToExistingLandblock(0xAAAAFFFFu, new[]
+        {
+            MakeStubEntity(1),
+            MakeStubEntity(2),
+        });
+
+        // Nothing in the flat view yet.
+        Assert.Empty(state.Entities);
+        Assert.Equal(2, state.PendingLiveEntityCount);
+
+        // Now load the landblock — pending entities should merge in.
+        state.AddLandblock(MakeStubLandblock(0xAAAAFFFFu));
+        Assert.Equal(2, state.Entities.Count);
+    }
+}