fix(battle-node): BattleSession crash safety, fresh-key per push, phase guards

- Wrap HandleMsgEventAsync / HandleAliveEventAsync bodies in try/catch(Exception)
  logging at Error, eliminating async-void unobserved-exception crash risk (Issue 1).
- Replace deterministic seq-based key generator with RandomNumberGenerator.GetInt32
  so each EncodeAndSendAsync call uses a fresh random key (Issue 2).
- Add `when Phase == …` guards to InitNetwork / Loaded / Swap cases in
  ComputeResponses; add default arm that logs+drops out-of-order URIs (Issue 3).
- Widen SendSioAckAsync arg from int to long; drop (int) cast at call site;
  boundary cast to int is now checked() for defensive overflow detection (Issue 4).
- Update RunAsync doc comment (was stale Task-13 placeholder) (Issue 5).
- Add Kill and out-of-order-Swap-before-Loaded tests (Issue 6).

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

SVSim.UnitTests/BattleNode/Sessions/BattleSessionDispatchTests.cs

@@ -61,4 +61,25 @@ public class BattleSessionDispatchTests
         Assert.That(noStock, Is.True);
         Assert.That(s.Phase, Is.EqualTo(BattleSessionPhase.Terminal));
     }
+
+    [Test]
+    public void Kill_PushesBattleFinishNoContest_TransitionsToTerminal()
+    {
+        var s = NewSession();
+        var responses = s.ComputeResponses(NewEnvelope(NetworkBattleUri.Kill));
+        var (env, noStock) = responses.Single();
+        Assert.That(env.Uri, Is.EqualTo(NetworkBattleUri.BattleFinish));
+        Assert.That(noStock, Is.True);
+        Assert.That(s.Phase, Is.EqualTo(BattleSessionPhase.Terminal));
+    }
+
+    [Test]
+    public void Swap_ArrivingBeforeLoaded_ProducesNoResponseAndDoesNotAdvancePhase()
+    {
+        var s = NewSession();
+        // Skip Loaded — fire Swap straight out of AwaitingInitNetwork.
+        var responses = s.ComputeResponses(NewEnvelope(NetworkBattleUri.Swap));
+        Assert.That(responses, Is.Empty);
+        Assert.That(s.Phase, Is.EqualTo(BattleSessionPhase.AwaitingInitNetwork));
+    }
 }