Fix late joiners never seeing earlier peers behind the paint reveal gate

Applying a snapshot now also raises the replica's sequence horizon: a
snapshot covers all events up to its captured cutoff. Previously the
horizon only advanced with the live stamp stream, so the server replica
of a player who never painted served snapshots with cutoff 0 - late
joiners then waited forever for the published baseline cutoff and the
reveal gate never opened (third client could not see the second).

Co-Authored-By: Claude Fable 5 <noreply@anthropic.com>

Assets/Scripts/Network/PlayerPaintSync.cs

@@ -263,6 +263,11 @@ public class PlayerPaintSync : NetworkBehaviour
             _snapReadyToApply = false;
             _painter.ApplyPaintSnapshot(_snapBuffer, _snapBaselineId);
             if (_snapCapturedSeq > _appliedCutoff) _appliedCutoff = _snapCapturedSeq; // receiver-side reveal gate
+            // The snapshot accounts for every event up to its cutoff, so raise this replica's horizon too.
+            // The SERVER replica serves late-join snapshots with _seqHorizon as the cutoff; without this a
+            // baseline that arrived AS a snapshot (owner publish, no live stamps since) would be served with
+            // a stale cutoff below the owner's published baseline, permanently reveal-gating the joiner.
+            if (_snapCapturedSeq > _seqHorizon) _seqHorizon = _snapCapturedSeq;
             if (logSync) LogSync($"applied snapshot baseline {_snapBaselineId}; replaying buffered events");
             FlushBuffered();
             _snapBuffer = null;