feat(runtime): expose dormant placement receipts

tests/AcDream.Runtime.Tests/Physics/RuntimeSetPositionStateTests.cs

@@ -103,6 +103,52 @@ public sealed class RuntimeSetPositionStateTests
         Assert.Equal(1, ownership.PreparedMoverCount);
     }
 
+    [Fact]
+    public void PublicPlacementChannelObservesRetriesAndAcknowledgesExactToken()
+    {
+        PhysicsEngine engine = FlatEngine(SourceLandblock, 0f);
+        using var lifetime = new RuntimeEntityObjectLifetime(engine);
+        var generation = new RuntimeGenerationToken(7UL);
+        lifetime.BindEventContext(() => generation, static () => 11UL);
+        RuntimeEntityRecord record = CreateRecord(lifetime, 0x70001012u, 1);
+        _ = AttachBody(lifetime, record, SourceCell);
+        var observer = new PlacementObserver();
+        using IDisposable subscription = lifetime.Placements.Subscribe(observer);
+
+        RuntimeSetPositionOutcome outcome = lifetime.Physics.SetPosition.Apply(
+            record,
+            record.PositionAuthorityVersion,
+            Command(Request(SourceCell, new Vector3(14f, 19f, 7f))));
+
+        Assert.Single(observer.Deltas);
+        Assert.True(lifetime.Placements.TryPeek(generation, out var pending));
+        Assert.Equal(outcome.Projection, pending.Token);
+        Assert.True(lifetime.Placements.RetryPending(generation));
+        Assert.Equal(2, observer.Deltas.Count);
+        Assert.Equal(
+            observer.Deltas[0].Placement,
+            observer.Deltas[1].Placement);
+
+        RuntimePlacementProjectionToken stale = outcome.Projection with
+        {
+            Revision = outcome.Projection.Revision + 1UL,
+        };
+        Assert.False(lifetime.Placements.Acknowledge(generation, stale));
+        Assert.False(lifetime.Placements.RetryPending(
+            new RuntimeGenerationToken(8UL)));
+        Assert.False(lifetime.Placements.TryPeek(
+            new RuntimeGenerationToken(8UL),
+            out _));
+        Assert.True(lifetime.Placements.Acknowledge(
+            generation,
+            outcome.Projection));
+        Assert.Equal(0, lifetime.Placements.PendingCount);
+        Assert.False(lifetime.Placements.TryPeek(generation, out _));
+        Assert.False(lifetime.Placements.Acknowledge(
+            generation,
+            outcome.Projection));
+    }
+
     [Fact]
     public void InWorldSetPositionPreservesObjectClockEpochPendingTimeAndActiveState()
     {