refactor(battle-node): move session phase + post-swap hands into BattleSessionState

SVSim.BattleNode/Sessions/Dispatch/BattleSessionState.cs

@@ -0,0 +1,11 @@
+namespace SVSim.BattleNode.Sessions.Dispatch;
+
+/// <summary>Mutable per-session state shared across frame handlers. Today: the session-level
+/// phase (only ever advanced to <see cref="BattleSessionPhase.Terminal"/>) and the mulligan
+/// barrier's post-swap hands. FUTURE (PvP equivalency, NOT this refactor): per-side idx->cardId
+/// maps + reveal-gating state land here.</summary>
+internal sealed class BattleSessionState
+{
+    public BattleSessionPhase SessionPhase { get; set; } = BattleSessionPhase.AwaitingInitNetwork;
+    public Dictionary<IBattleParticipant, long[]> PostSwapHands { get; } = new();
+}