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/Sessions/BattleSession.cs

@@ -209,10 +209,25 @@ public sealed class BattleSession
                 Phase = BattleSessionPhase.AwaitingSwap;
                 break;
             case NetworkBattleUri.Swap when Phase == BattleSessionPhase.AwaitingSwap:
-                result.Add((ScriptedLifecycle.BuildSwapResponse(ExtractIdxList(env)), NoStock: false));
-                result.Add((ScriptedLifecycle.BuildReady(), NoStock: false));
+            {
+                // Compute the actual post-mulligan hand: any idx in idxList that's in the initial
+                // 3-card hand gets replaced with a fresh deck idx. Both Swap response AND Ready
+                // need the SAME hand — the client diffs them to compute "drawn cards" and errors
+                // out with "Card swap failed: AbandonCards[...]/DrawCards[]" if they don't agree.
+                var hand = ScriptedLifecycle.ComputeHandAfterSwap(ExtractIdxList(env));
+                result.Add((ScriptedLifecycle.BuildSwapResponse(hand), NoStock: false));
+                result.Add((ScriptedLifecycle.BuildReady(hand), NoStock: false));
                 Phase = BattleSessionPhase.AfterReady;
                 break;
+            }
+            case NetworkBattleUri.TurnEnd when Phase == BattleSessionPhase.AfterReady:
+            case NetworkBattleUri.TurnEndFinal when Phase == BattleSessionPhase.AfterReady:
+                // Push a minimal opponent TurnStart so the client transitions to "Opponent's turn…"
+                // display. v1 doesn't simulate the opponent — once this lands, the client sits
+                // there indefinitely (this IS the documented v1 stopping point).
+                result.Add((ScriptedLifecycle.BuildOpponentTurnStart(), NoStock: false));
+                Phase = BattleSessionPhase.OpponentTurn;
+                break;
             case NetworkBattleUri.Retire:
             case NetworkBattleUri.Kill:
                 // These always terminate, regardless of phase.

SVSim.BattleNode/Sessions/BattleSessionPhase.cs

@@ -11,5 +11,6 @@ public enum BattleSessionPhase
     AwaitingLoaded,
     AwaitingSwap,
     AfterReady,
+    OpponentTurn,
     Terminal,
 }