feat(battle-node): capture session-scoped CT in BattleSession.RunAsync

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -18,6 +18,19 @@ public sealed class BattleSession
     private readonly WebSocket _ws;
     private readonly ILogger<BattleSession> _log;
 
+    /// <summary>
+    /// Cancellation token for the session lifetime, captured from <see cref="RunAsync"/>'s
+    /// parameter. Read by all send helpers so host shutdown promptly terminates pending
+    /// writes (was <c>CancellationToken.None</c> on every send pre-this-slice).
+    /// </summary>
+    /// <remarks>
+    /// Defaults to <see cref="CancellationToken.None"/> when <see cref="RunAsync"/> hasn't
+    /// run — e.g. unit tests that drive <see cref="ComputeResponses"/> directly without
+    /// going through the WS pump. None of those tests exercise send code, so the default
+    /// is safe.
+    /// </remarks>
+    private CancellationToken _sessionCt;
+
     public string BattleId { get; }
     public long ViewerId { get; }
     public BattleSessionPhase Phase { get; internal set; } = BattleSessionPhase.AwaitingInitNetwork;
@@ -37,6 +50,7 @@ public sealed class BattleSession
     /// </summary>
     public async Task RunAsync(CancellationToken cancellation)
     {
+        _sessionCt = cancellation;
         await SendEioOpenAsync(cancellation);
 
         var buffer = new byte[8192];