feat(battle-node): InboundTracker dedupes client pubSeq + tracks high-water

SVSim.UnitTests/BattleNode/Reliability/InboundTrackerTests.cs

@@ -0,0 +1,41 @@
+using NUnit.Framework;
+using SVSim.BattleNode.Reliability;
+
+namespace SVSim.UnitTests.BattleNode.Reliability;
+
+[TestFixture]
+public class InboundTrackerTests
+{
+    [Test]
+    public void Observe_FirstSeenPubSeq_ReturnsDispatchTrue()
+    {
+        var t = new InboundTracker();
+        Assert.That(t.Observe(pubSeq: 1), Is.True);
+    }
+
+    [Test]
+    public void Observe_SamePubSeqTwice_SecondReturnsFalse()
+    {
+        var t = new InboundTracker();
+        t.Observe(1);
+        Assert.That(t.Observe(1), Is.False);
+    }
+
+    [Test]
+    public void Observe_DifferentPubSeqs_BothDispatch()
+    {
+        var t = new InboundTracker();
+        Assert.That(t.Observe(1), Is.True);
+        Assert.That(t.Observe(2), Is.True);
+    }
+
+    [Test]
+    public void HighWaterMark_TracksHighestObserved()
+    {
+        var t = new InboundTracker();
+        t.Observe(3);
+        t.Observe(1);
+        t.Observe(5);
+        Assert.That(t.HighWaterMark, Is.EqualTo(5));
+    }
+}