feat(battle-node): add BattleType, BattleFinishReason, BattlePlayer

Phase 1 foundation types for the v2 broker architecture. Nothing uses
them yet; they land alongside the existing v1.2 code so subsequent
tasks can extract the participant interface and impls.

SVSim.BattleNode/Sessions/BattleFinishReason.cs

@@ -0,0 +1,13 @@
+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>Reason a participant was terminated. Carried to
+/// <see cref="IBattleParticipant.TerminateAsync"/> so impls can log/clean differently
+/// per cause. Cleanup itself is the same regardless of reason.</summary>
+public enum BattleFinishReason
+{
+    NormalFinish,
+    Retire,
+    OpponentDisconnect,
+    Timeout,
+    ServerAbort,
+}