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) =>

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

@@ -0,0 +1,15 @@
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.BattleNode.Sessions.Dispatch.Handlers;
+
+internal sealed class TurnStartHandler : IFrameHandler
+{
+    public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
+    {
+        // Forward the opponent's turn-open to the other side. Union of the two legacy arms:
+        // BothAfterReady (PvP / scripted real player) OR a scripted-bot emission (test stub path).
+        if (ctx.BothAfterReady() || ctx.IsScriptedBot(ctx.From))
+            return new[] { new DispatchRoute(ctx.Other, ctx.Env, false) };
+        return Array.Empty<DispatchRoute>();
+    }
+}