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/TurnEndFinalHandler.cs

@@ -7,13 +7,13 @@ internal sealed class TurnEndFinalHandler : IFrameHandler
     public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
     {
         // case 4: Bot — Judge to sender only.
-        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 9: general — forward the envelope to other + paired BattleFinish + Terminal.
-        if (ctx.SenderPhase == BattleSessionPhase.AfterReady)
+        if (ctx.SenderPhase == HandshakePhase.AfterReady)
         {
-            ctx.State.SessionPhase = BattleSessionPhase.Terminal;
+            ctx.State.Lifecycle = SessionLifecycle.Terminal;
             // Polarity: the SENDER dealt the lethal, so From WINS / Other LOSES. This is the
             // OPPOSITE of RetireKillHandler (From LOSES there — retire is self-inflicted).
             // Intentional — do NOT "consistency-fix" the two handlers to match; a swap here