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/Participants/RealParticipant.cs

@@ -19,7 +19,7 @@ namespace SVSim.BattleNode.Sessions.Participants;
 /// </summary>
 internal interface IHasHandshakePhase
 {
-    BattleSessionPhase Phase { get; set; }
+    HandshakePhase Phase { get; set; }
 }
 
 /// <summary>
@@ -66,9 +66,9 @@ public sealed class RealParticipant : IBattleParticipant, IHasHandshakePhase
     /// because they never send the gating URIs. Also satisfies
     /// <see cref="IHasHandshakePhase"/> (the interface BattleSession uses to gate
     /// handshake dispatch without depending on the concrete RealParticipant type).</summary>
-    internal BattleSessionPhase Phase { get; set; } = BattleSessionPhase.AwaitingInitNetwork;
+    internal HandshakePhase Phase { get; set; } = HandshakePhase.AwaitingInitNetwork;
 
-    BattleSessionPhase IHasHandshakePhase.Phase
+    HandshakePhase IHasHandshakePhase.Phase
     {
         get => Phase;
         set => Phase = value;