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/Bridge/BattlePlayer.cs

@@ -0,0 +1,5 @@
+namespace SVSim.BattleNode.Bridge;
+
+/// <summary>One player slot for a pending battle. Carries the viewer's identity and
+/// the per-battle MatchContext snapshot built at do_matching time.</summary>
+public sealed record BattlePlayer(long ViewerId, MatchContext Context);