fix(battle-node): real mulligan card replacement + opponent TurnStart push

Two issues caught during v1 smoke at the mulligan / first-turn boundary:

1) BuildSwapResponse ignored the player's idxList and echoed the same
   3-card hand back. The client diffs the new self[] against the Deal
   to compute "drawn cards" — empty diff against the same hand throws
   "Card swap failed: AbandonCards[X]/DrawCards[]". Replace swapped
   idxs with fresh deck idxs (initial hand was 1/2/3, deck has 4..30
   still available). Same hand must flow into Ready since the client
   diffs again there. Move the hand computation into a new helper
   ComputeHandAfterSwap and have ComputeResponses thread it through
   both BuildSwapResponse and BuildReady.

2) The client doesn't transition to the "Opponent's turn…" display
   on its own after sending TurnEnd — it waits for the server to push
   an opponent TurnStart (per prod TK2 capture line 14). Without it
   the UI just sits on the end-of-turn frame. Add a TurnEnd handler
   that pushes a minimal TurnStart{spin} and transitions to a new
   OpponentTurn phase, which IS the documented v1 stopping point.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

SVSim.BattleNode/Lifecycle/ScriptedLifecycle.cs

@@ -96,45 +96,77 @@ public static class ScriptedLifecycle
         return EnvelopeForPush(NetworkBattleUri.Deal, body);
     }
 
-    public static MsgEnvelope BuildSwapResponse(IReadOnlyList<long> swapIndices)
+    /// <summary>
+    /// Initial 3-card hand idxs from <see cref="BuildDeal"/>. Each position in this array
+    /// is one card; the value is the card's deck idx.
+    /// </summary>
+    private static readonly long[] InitialHand = { 1, 2, 3 };
+
+    /// <summary>
+    /// Compute the player's hand after a mulligan. For every idx in <paramref name="swapIndices"/>
+    /// that is currently in the hand, replace it with the next unused deck idx (starting at 4,
+    /// since 1..3 were dealt). Positions of kept cards are preserved.
+    /// </summary>
+    public static long[] ComputeHandAfterSwap(IReadOnlyList<long> swapIndices)
+    {
+        var hand = (long[])InitialHand.Clone();
+        var nextDeckIdx = 4L;
+        for (var pos = 0; pos < hand.Length; pos++)
+        {
+            if (swapIndices.Contains(hand[pos]))
+            {
+                hand[pos] = nextDeckIdx++;
+            }
+        }
+        return hand;
+    }
+
+    public static MsgEnvelope BuildSwapResponse(IReadOnlyList<long> hand)
     {
-        // v1: ignore the player's chosen indices and echo the same hand back.
-        // (Acceptable because the client doesn't validate which idxs come back — it just renders them.)
-        _ = swapIndices;
         var body = new Dictionary<string, object?>
         {
-            ["self"] = new List<object?>
-            {
-                new Dictionary<string, object?> { ["pos"] = 0, ["idx"] = 1 },
-                new Dictionary<string, object?> { ["pos"] = 1, ["idx"] = 2 },
-                new Dictionary<string, object?> { ["pos"] = 2, ["idx"] = 3 },
-            },
+            ["self"] = BuildPosIdxList(hand),
         };
         return EnvelopeForPush(NetworkBattleUri.Swap, body);
     }
 
-    public static MsgEnvelope BuildReady()
+    public static MsgEnvelope BuildReady(IReadOnlyList<long> hand)
     {
         var body = new Dictionary<string, object?>
         {
-            ["self"] = new List<object?>
-            {
-                new Dictionary<string, object?> { ["pos"] = 0, ["idx"] = 1 },
-                new Dictionary<string, object?> { ["pos"] = 1, ["idx"] = 2 },
-                new Dictionary<string, object?> { ["pos"] = 2, ["idx"] = 3 },
-            },
-            ["oppo"] = new List<object?>
-            {
-                new Dictionary<string, object?> { ["pos"] = 0, ["idx"] = 1 },
-                new Dictionary<string, object?> { ["pos"] = 1, ["idx"] = 2 },
-                new Dictionary<string, object?> { ["pos"] = 2, ["idx"] = 3 },
-            },
+            ["self"] = BuildPosIdxList(hand),
+            // Opponent hand stays at the static 3 cards for v1.
+            ["oppo"] = BuildPosIdxList(InitialHand),
             ["idxChangeSeed"] = 771335280,
             ["spin"] = 243,
         };
         return EnvelopeForPush(NetworkBattleUri.Ready, body);
     }
 
+    /// <summary>
+    /// Generic TurnStart push used to transition the client into "Opponent's turn…" state
+    /// after the player's TurnEnd. v1 doesn't simulate the opponent — once this lands the
+    /// client sits at the opponent-turn display indefinitely.
+    /// </summary>
+    public static MsgEnvelope BuildOpponentTurnStart()
+    {
+        var body = new Dictionary<string, object?>
+        {
+            ["spin"] = 100,
+        };
+        return EnvelopeForPush(NetworkBattleUri.TurnStart, body);
+    }
+
+    private static List<object?> BuildPosIdxList(IReadOnlyList<long> hand)
+    {
+        var list = new List<object?>(hand.Count);
+        for (var pos = 0; pos < hand.Count; pos++)
+        {
+            list.Add(new Dictionary<string, object?> { ["pos"] = pos, ["idx"] = (int)hand[pos] });
+        }
+        return list;
+    }
+
     private static List<object?> BuildDummyDeck()
     {
         var deck = new List<object?>(30);