refactor(battlenode): split BattleSessionPhase into HandshakePhase + SessionLifecycle

Behavior-preserving; 231 BattleNode tests green.

One enum conflated two axes. Split:
- HandshakePhase (per participant): AwaitingInitNetwork..AfterReady. On
  IHasHandshakePhase.Phase, FrameDispatchContext.SenderPhase, the handler gates.
- SessionLifecycle (per battle): Active | Terminal. On the renamed
  BattleSessionState.Lifecycle (was SessionPhase, defaulting to a handshake value)
  and BattleSession.Lifecycle (was Phase). Reads are only != Terminal, so the
  Active default is behavior-identical.

OpponentTurn was dead (never assigned) -> dropped. BattleSessionPhase deleted; the
two axes can no longer be cross-assigned.

Co-Authored-By: Claude Opus 4.8 <noreply@anthropic.com>

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

@@ -8,12 +8,12 @@ internal sealed class TurnEndHandler : IFrameHandler
     public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
     {
         // case 4: Bot — Judge to sender only (no real opponent; client flips back to its local AI).
-        if (ctx.Type == BattleType.Bot && ctx.SenderPhase == BattleSessionPhase.AfterReady)
+        if (ctx.Type == BattleType.Bot && ctx.SenderPhase == HandshakePhase.AfterReady)
             return new[] { new DispatchRoute(ctx.From, BattleFrames.BuildJudgeBroadcast(), Stock.Normal) };
 
         // case 8: general AfterReady arm — PvP forwards a {turnState} TurnEnd to the opponent
         // (handover gate). Any non-Pvp non-Bot type that reaches AfterReady consumes the frame.
-        if (ctx.SenderPhase == BattleSessionPhase.AfterReady)
+        if (ctx.SenderPhase == HandshakePhase.AfterReady)
         {
             if (ctx.Type == BattleType.Pvp && ctx.BothAfterReady())
             {