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/InitBattleHandler.cs

@@ -8,18 +8,18 @@ internal sealed class InitBattleHandler : IFrameHandler
     public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
     {
         // case 2: Bot — ack only, NO Matched (Matched would corrupt client opponent info).
-        if (ctx.Type == BattleType.Bot && ctx.SenderPhase == BattleSessionPhase.AwaitingInitBattle)
+        if (ctx.Type == BattleType.Bot && ctx.SenderPhase == HandshakePhase.AwaitingInitBattle)
         {
             var r = new List<DispatchRoute>
             {
                 new(ctx.From, BattleFrames.BuildAck(NetworkBattleUri.InitBattle), Stock.Bypass),
             };
-            ctx.SenderPhase = BattleSessionPhase.AwaitingLoaded;
+            ctx.SenderPhase = HandshakePhase.AwaitingLoaded;
             return r;
         }
 
         // case 5: general — push Matched (per-perspective) to the sender only.
-        if (ctx.SenderPhase == BattleSessionPhase.AwaitingInitBattle)
+        if (ctx.SenderPhase == HandshakePhase.AwaitingInitBattle)
         {
             var r = new List<DispatchRoute>
             {
@@ -28,7 +28,7 @@ internal sealed class InitBattleHandler : IFrameHandler
                     ctx.BattleId, BattleSeeds.Stable(ctx.State.MasterSeed),
                     ctx.State.GetShuffledDeck(ctx.From)), Stock.Normal),
             };
-            ctx.SenderPhase = BattleSessionPhase.AwaitingLoaded;
+            ctx.SenderPhase = HandshakePhase.AwaitingLoaded;
             return r;
         }