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/SwapHandler.cs

@@ -8,7 +8,7 @@ internal sealed class SwapHandler : IFrameHandler
 {
     public IReadOnlyList<DispatchRoute> Handle(FrameDispatchContext ctx)
     {
-        if (ctx.SenderPhase != BattleSessionPhase.AwaitingSwap)
+        if (ctx.SenderPhase != HandshakePhase.AwaitingSwap)
             return Array.Empty<DispatchRoute>();
 
         var routes = new List<DispatchRoute>();
@@ -17,7 +17,7 @@ internal sealed class SwapHandler : IFrameHandler
         // SwapResponse is always immediate — completes the sender's own mulligan UI.
         routes.Add(new DispatchRoute(ctx.From, ServerBattleFrames.BuildSwapResponse(hand), Stock.Normal));
         ctx.State.PostSwapHands[ctx.From] = hand;
-        ctx.SenderPhase = BattleSessionPhase.AfterReady;
+        ctx.SenderPhase = HandshakePhase.AfterReady;
 
         // Release Ready to every swapper once all handshake-driving participants have swapped.
         // IHasHandshakePhase membership IS the "participates in mulligan" set.