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/FrameDispatchContext.cs

@@ -20,7 +20,7 @@ internal sealed class FrameDispatchContext
 
     /// <summary>The dispatching participant's handshake phase (null for a non-IHasHandshakePhase
     /// participant, e.g. NoOpBot). Setting it advances the sender.</summary>
-    internal BattleSessionPhase? SenderPhase
+    internal HandshakePhase? SenderPhase
     {
         get => (From as IHasHandshakePhase)?.Phase;
         set { if (From is IHasHandshakePhase p && value is { } v) p.Phase = v; }
@@ -29,6 +29,6 @@ internal sealed class FrameDispatchContext
     /// <summary>Both participants have completed the handshake. Reads A/B (not From/Other) so the
     /// result is identical regardless of which side sent the frame — matches legacy BothAfterReady.</summary>
     internal bool BothAfterReady() =>
-        (A as IHasHandshakePhase)?.Phase == BattleSessionPhase.AfterReady &&
-        (B as IHasHandshakePhase)?.Phase == BattleSessionPhase.AfterReady;
+        (A as IHasHandshakePhase)?.Phase == HandshakePhase.AfterReady &&
+        (B as IHasHandshakePhase)?.Phase == HandshakePhase.AfterReady;
 }