fix(streaming): #138 — rescue persistent entities from the pending bucket on unload

GpuWorldState.RemoveLandblock rescued persistent entities (the player)
only from the _loaded list, silently dropping one sitting in the
_pendingByLandblock bucket. The player is re-injected via AppendLiveEntity
every frame; right after a teleport its destination landblock has not
streamed in yet, so the player lands in the pending bucket — and if that
landblock is then unloaded during the streaming churn, the persistent
entry was dropped, violating the "persistent therefore survives unload"
contract. Leading candidate for the #138 "own avatar vanishes after a
couple round-trips" symptom (cumulative; needs user visual confirm).

Fix: scan the pending bucket for persistent guids and rescue them too,
so DrainRescued re-parks them at the next valid landblock. Provable
correctness fix with a deterministic test (rescue-from-pending plus a
negative for non-persistent). Core tests green.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

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

@@ -137,4 +137,51 @@ public class GpuWorldStateTests
         state.AddLandblock(MakeStubLandblock(0xA9B4FFFFu));
         Assert.True(state.IsLoaded(0xA9B4FFFFu));
     }
+
+    private static WorldEntity MakeServerEntity(uint id, uint serverGuid)
+        => new()
+        {
+            Id = id,
+            ServerGuid = serverGuid,
+            SourceGfxObjOrSetupId = 0x01000001u,
+            Position = System.Numerics.Vector3.Zero,
+            Rotation = System.Numerics.Quaternion.Identity,
+            MeshRefs = Array.Empty<MeshRef>(),
+        };
+
+    [Fact]
+    public void RemoveLandblock_RescuesPersistentEntity_FromPendingBucket()
+    {
+        // #138 (secondary): the player is re-injected via AppendLiveEntity every
+        // frame; right after a teleport its landblock hasn't streamed in, so it
+        // lands in the pending bucket. If that landblock is then unloaded before
+        // it finishes loading, the persistent entity must still be rescued —
+        // otherwise the avatar vanishes after a couple round-trips.
+        var state = new GpuWorldState();
+        const uint playerGuid = 0x50000001u;
+        state.MarkPersistent(playerGuid);
+
+        var player = MakeServerEntity(id: 1, serverGuid: playerGuid);
+        state.AppendLiveEntity(0xA9B40011u, player);   // landblock not loaded → pending
+        Assert.Equal(1, state.PendingLiveEntityCount);
+
+        state.RemoveLandblock(0xA9B4FFFFu);            // unloaded while still pending
+
+        Assert.Contains(player, state.DrainRescued()); // rescued, not dropped
+    }
+
+    [Fact]
+    public void RemoveLandblock_DoesNotRescue_NonPersistentPendingEntity()
+    {
+        // A normal server object (door) in the pending bucket is NOT persistent;
+        // it must still be dropped (and is recoverable via #138 re-hydrate from
+        // _lastSpawnByGuid, not via the rescue path).
+        var state = new GpuWorldState();
+        var door = MakeServerEntity(id: 2, serverGuid: 0x7A9B4001u);  // not marked persistent
+        state.AppendLiveEntity(0xA9B40011u, door);
+
+        state.RemoveLandblock(0xA9B4FFFFu);
+
+        Assert.Empty(state.DrainRescued());
+    }
 }