feat(app): observe canonical placement receipts

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

@@ -173,6 +173,28 @@ public sealed class RuntimePlacementProjectionSubscriptionTests
         Assert.False(subscription.HasAppliedReceiptAwaitingAcknowledgement);
     }
 
+    [Fact]
+    public void SubscribeWithoutDrainPublishesOnlyAfterExplicitRetry()
+    {
+        using var fixture = new Fixture();
+        RuntimeSetPositionOutcome pending = fixture.Place(
+            fixture.First,
+            new Vector3(18.5f, 18f, 7f));
+        var sink = new RecordingSink();
+
+        using var subscription = fixture.Subscribe(
+            sink,
+            retryPendingOnSubscribe: false);
+
+        Assert.Empty(sink.Applied);
+        Assert.Equal(1, fixture.Lifetime.Placements.PendingCount);
+
+        Assert.True(subscription.RetryPending());
+
+        Assert.Equal(pending.Projection, Assert.Single(sink.Applied).Token);
+        Assert.Equal(0, fixture.Lifetime.Placements.PendingCount);
+    }
+
     [Fact]
     public void SyntheticWithdrawalUsesTheSameExactSinkAndAckPath()
     {
@@ -263,10 +285,12 @@ public sealed class RuntimePlacementProjectionSubscriptionTests
         internal RuntimeEntityRecord? Second { get; }
 
         internal RuntimePlacementProjectionSubscription Subscribe(
-            IRuntimePlacementProjectionSink sink) => new(
+            IRuntimePlacementProjectionSink sink,
+            bool retryPendingOnSubscribe = true) => new(
                 Lifetime.Placements,
                 () => _generation,
-                sink);
+                sink,
+                retryPendingOnSubscribe);
 
         internal RuntimeSetPositionOutcome Place(
             RuntimeEntityRecord record,