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/SessionLifecycle.cs

@@ -0,0 +1,15 @@
+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>
+/// Session-global lifecycle. A battle stays <see cref="Active"/> until a terminal event — a lethal
+/// TurnEndFinal, a Retire/Kill, or the disconnect drop cascade — flips it to <see cref="Terminal"/>,
+/// after which the drop cascade will not synthesize another BattleFinish. Distinct from the
+/// per-participant <see cref="HandshakePhase"/> (which side reached which setup step); this is one
+/// axis per battle. Only these two states are load-bearing — the handshake progression lives on the
+/// other enum.
+/// </summary>
+public enum SessionLifecycle
+{
+    Active,
+    Terminal,
+}