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);

SVSim.BattleNode/Sessions/BattleFinishReason.cs

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+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,
+}

SVSim.BattleNode/Sessions/BattleType.cs

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+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>
+/// Discriminator for a pending battle and the session it produces. See
+/// docs/superpowers/specs/2026-06-01-battle-node-v2-architecture-design.md.
+/// </summary>
+public enum BattleType
+{
+    /// <summary>Two real players. Server brokers between two WebSockets.
+    /// Both <c>BattlePlayer</c> slots required.</summary>
+    Pvp,
+
+    /// <summary>One real player; opponent runs in the client (prod's IsAINetwork
+    /// path; matched only in rank rotation / rank unlimited per prod). Server is
+    /// ack-only. <c>p2</c> must be null.</summary>
+    Bot,
+
+    /// <summary>One real player; server scripts the opponent (today's v1.2
+    /// behaviour, preserved as a solo testing harness). <c>p2</c> currently null;
+    /// future server-driven bot config can ride on <c>p2</c>.</summary>
+    Scripted,
+}