refactor(battle-node): extract TurnStartHandler

Unions the two legacy TurnStart arms (IsRealForwardableFromScripted case 11 +
BothAfterReady case 12) into TurnStartHandler. Both arms produce (Other, Env, false)
with no extra guards or state mutations — union is behavior-equivalent.

Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>

SVSim.BattleNode/Sessions/BattleSession.cs

@@ -44,6 +44,7 @@ public sealed class BattleSession
             [NetworkBattleUri.TurnEndFinal] = new TurnEndFinalHandler(),
             [NetworkBattleUri.Retire] = new RetireKillHandler(),
             [NetworkBattleUri.Kill] = new RetireKillHandler(),
+            [NetworkBattleUri.TurnStart] = new TurnStartHandler(),
         };
 
     private FrameDispatchContext BuildContext(IBattleParticipant from, MsgEnvelope env) =>