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

SVSim.BattleNode/Reliability/InboundTracker.cs

@@ -0,0 +1,20 @@
+namespace SVSim.BattleNode.Reliability;
+
+/// <summary>
+/// Per-session inbound-emit ledger. Dedupes the client's pubSeq so we never dispatch
+/// a retransmitted emit twice; ack-echo (via SIO callback) is the caller's job.
+/// </summary>
+public sealed class InboundTracker
+{
+    private readonly HashSet<long> _seen = new();
+
+    /// <summary>Highest pubSeq observed so far. Reported via Gungnir for diagnostics.</summary>
+    public long HighWaterMark { get; private set; }
+
+    /// <summary>Record an incoming pubSeq. Returns true if the caller should dispatch the envelope, false on duplicate.</summary>
+    public bool Observe(long pubSeq)
+    {
+        if (pubSeq > HighWaterMark) HighWaterMark = pubSeq;
+        return _seen.Add(pubSeq);
+    }
+}