feat(battle-node): derive Matched.seed + Ready.idxChangeSeed from master seed

InitBattle now emits Stable(master) as the shared effect seed and the master-
shuffled deck as selfDeck; Swap emits each recipient's per-side IdxChange seed.
BattleSession exposes + logs the master seed per battle for future replay.
Updated lifecycle/dispatch/integration tests (deck assertions now permutation-
based since selfDeck is shuffled).

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -22,6 +22,10 @@ public sealed class BattleSession
 
     private readonly BattleSessionState _state = new();
 
+    /// <summary>The per-battle master seed (see <see cref="BattleSessionState.MasterSeed"/>).
+    /// Exposed for logging + future replay persistence.</summary>
+    public int MasterSeed => _state.MasterSeed;
+
     public string BattleId { get; }
     public BattleType Type { get; }
     public IBattleParticipant A { get; }
@@ -71,6 +75,8 @@ public sealed class BattleSession
         B = b;
         _log = log;
 
+        _log.LogInformation("BattleSession {Bid}: master seed {Seed}", BattleId, _state.MasterSeed);
+
         // Subscribe to both participants' emissions.
         A.FrameEmitted += OnFrameFromA;
         B.FrameEmitted += OnFrameFromB;