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/HandshakePhase.cs

@@ -0,0 +1,18 @@
+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>
+/// Per-participant progression through the v1 server-authored setup handshake. Each side advances
+/// InitNetwork → InitBattle → Loaded → Swap → AfterReady as the session acks its emits. Tracked
+/// per participant via <see cref="Participants.IHasHandshakePhase"/>; the session reads the
+/// SENDER's phase (<see cref="Dispatch.FrameDispatchContext.SenderPhase"/>) to gate which setup
+/// frame to author next. Distinct from the session-global <see cref="SessionLifecycle"/> — this is
+/// one axis per side, that is one axis per battle.
+/// </summary>
+public enum HandshakePhase
+{
+    AwaitingInitNetwork,
+    AwaitingInitBattle,
+    AwaitingLoaded,
+    AwaitingSwap,
+    AfterReady,
+}