feat(battle-node): per-battle master seed + node-side deck shuffle

GetOrSeedDeckMap now seeds from a Fisher-Yates shuffle of the deck keyed by the
per-battle MasterSeed, so the reveal map and the wire selfDeck share one
shuffled order. Updated the existing build-order test to the shuffle semantics.

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

SVSim.BattleNode/Sessions/Dispatch/BattleSessionState.cs

@@ -1,3 +1,4 @@
+using SVSim.BattleNode.Lifecycle;
 using SVSim.BattleNode.Sessions;
 
 namespace SVSim.BattleNode.Sessions.Dispatch;
@@ -9,6 +10,34 @@ namespace SVSim.BattleNode.Sessions.Dispatch;
 /// <see cref="IdxToCardId"/> map via <see cref="RecordToken"/>; a reveal-gate set is still future.</summary>
 internal sealed class BattleSessionState
 {
+    /// <summary>The one random value chosen per battle. Every per-battle RNG (shared effect seed,
+    /// each side's deck shuffle + idxChangeSeed) derives from it via <see cref="BattleSeeds"/>.
+    /// Logged at session start so a battle's randomness is reproducible (future replay).</summary>
+    public int MasterSeed { get; }
+
+    /// <param name="masterSeed">Test hook — production uses the random default.</param>
+    public BattleSessionState(int? masterSeed = null) =>
+        MasterSeed = masterSeed ?? Random.Shared.Next();
+
+    private readonly Dictionary<IBattleParticipant, IReadOnlyList<long>> _shuffledDecks = new();
+
+    /// <summary>This side's deck, shuffled deterministically from <see cref="MasterSeed"/>
+    /// (Fisher–Yates). Cached per side. Both the wire selfDeck (Matched) and the reveal map
+    /// (<see cref="GetOrSeedDeckMap"/>) read this, so they share one shuffled order.</summary>
+    public IReadOnlyList<long> GetShuffledDeck(IBattleParticipant side)
+    {
+        if (_shuffledDecks.TryGetValue(side, out var cached)) return cached;
+        var deck = side.Context.SelfDeckCardIds.ToArray();
+        var rng = new Random(BattleSeeds.DeckShuffle(MasterSeed, side.ViewerId));
+        for (var i = deck.Length - 1; i > 0; i--)
+        {
+            var j = rng.Next(i + 1);
+            (deck[i], deck[j]) = (deck[j], deck[i]);
+        }
+        _shuffledDecks[side] = deck;
+        return deck;
+    }
+
     public BattleSessionPhase SessionPhase { get; set; } = BattleSessionPhase.AwaitingInitNetwork;
     public Dictionary<IBattleParticipant, long[]> PostSwapHands { get; } = new();
 
@@ -17,14 +46,15 @@ internal sealed class BattleSessionState
     /// from add ops via <see cref="RecordToken"/>).</summary>
     public Dictionary<IBattleParticipant, Dictionary<int, long>> IdxToCardId { get; } = new();
 
-    /// <summary>The sender's idx->cardId map, seeding it from its <see cref="MatchContext"/> on first
-    /// use. <c>BuildPlayerDeck</c> assigns deck idx = position+1, so entry (i+1) -> cardIds[i].</summary>
+    /// <summary>The sender's idx->cardId map, seeding it from its <see cref="GetShuffledDeck"/> order on
+    /// first use. Deck idx = position+1 in the shuffled order, so entry (i+1) -> shuffledDeck[i]. The
+    /// wire selfDeck (Matched) is built from the same shuffled order, so the two agree.</summary>
     public IReadOnlyDictionary<int, long> GetOrSeedDeckMap(IBattleParticipant side)
     {
         if (!IdxToCardId.TryGetValue(side, out var map))
         {
             map = new Dictionary<int, long>();
-            var deck = side.Context.SelfDeckCardIds;
+            var deck = GetShuffledDeck(side);
             for (var i = 0; i < deck.Count; i++) map[i + 1] = deck[i];
             IdxToCardId[side] = map;
         }