fix(battle-node): MatchingBridge avoids Math.Abs(int.MinValue) overflow

Cast GetHashCode() result to long before Math.Abs to prevent OverflowException
on the ~1-in-4B case where GetHashCode returns int.MinValue. Adds a regression
test pinning the 12-digit decimal format end-to-end.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.UnitTests/BattleNode/Bridge/MatchingBridgeTests.cs

@@ -32,4 +32,15 @@ public class MatchingBridgeTests
 
         Assert.That(a.BattleId, Is.Not.EqualTo(b.BattleId));
     }
+
+    [Test]
+    public void RegisterPendingBattle_ProducesTwelveDigitDecimalBattleId()
+    {
+        var bridge = new MatchingBridge(new InMemoryBattleSessionStore(), new BattleNodeOptions());
+
+        var match = bridge.RegisterPendingBattle(viewerId: 1);
+
+        Assert.That(match.BattleId, Has.Length.EqualTo(12));
+        Assert.That(match.BattleId, Does.Match("^[0-9]{12}$"));
+    }
 }