refactor(battle-node): extract SwapHandler (mulligan barrier)

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -39,6 +39,7 @@ public sealed class BattleSession
             [NetworkBattleUri.InitNetwork] = new InitNetworkHandler(),
             [NetworkBattleUri.InitBattle] = new InitBattleHandler(),
             [NetworkBattleUri.Loaded] = new LoadedHandler(),
+            [NetworkBattleUri.Swap] = new SwapHandler(),
         };
 
     private FrameDispatchContext BuildContext(IBattleParticipant from, MsgEnvelope env) =>

SVSim.BattleNode/Sessions/Dispatch/Handlers/SwapHandler.cs

@@ -0,0 +1,39 @@
+using SVSim.BattleNode.Lifecycle;
+using SVSim.BattleNode.Protocol;
+using SVSim.BattleNode.Sessions.Participants; // IHasHandshakePhase
+
+namespace SVSim.BattleNode.Sessions.Dispatch.Handlers;
+
+internal sealed class SwapHandler : IFrameHandler
+{
+    public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
+    {
+        if (ctx.SenderPhase != BattleSessionPhase.AwaitingSwap)
+            return Array.Empty<DispatchRoute>();
+
+        var routes = new List<DispatchRoute>();
+        var hand = ScriptedLifecycle.ComputeHandAfterSwap(BattleFrames.ExtractIdxList(ctx.Env));
+
+        // SwapResponse is always immediate — completes the sender's own mulligan UI.
+        routes.Add(new DispatchRoute(ctx.From, ScriptedLifecycle.BuildSwapResponse(hand), false));
+        ctx.State.PostSwapHands[ctx.From] = hand;
+        ctx.SenderPhase = BattleSessionPhase.AfterReady;
+
+        // Release Ready to every swapper once all handshake-driving participants have swapped.
+        // IHasHandshakePhase membership IS the "participates in mulligan" set.
+        var swappers = new[] { ctx.A, ctx.B }.Where(p => p is IHasHandshakePhase).ToList();
+        if (swappers.All(ctx.State.PostSwapHands.ContainsKey))
+        {
+            foreach (var p in swappers)
+            {
+                var opponent = ReferenceEquals(p, ctx.A) ? ctx.B : ctx.A;
+                var ready = opponent is IHasHandshakePhase
+                            && ctx.State.PostSwapHands.TryGetValue(opponent, out var oppoHand)
+                    ? ScriptedLifecycle.BuildReady(ctx.State.PostSwapHands[p], oppoHand)
+                    : ScriptedLifecycle.BuildReady(ctx.State.PostSwapHands[p]);
+                routes.Add(new DispatchRoute(p, ready, false));
+            }
+        }
+        return routes;
+    }
+}