feat(battle-node): add IBattleParticipant interface

Central abstraction for v2 broker. PushAsync (session -> participant),
FrameEmitted (participant -> session), RunAsync (drives inbound),
TerminateAsync (cleanup). Three impls land in Tasks 3-5.

SVSim.BattleNode/Sessions/IBattleParticipant.cs

@@ -0,0 +1,46 @@
+using SVSim.BattleNode.Bridge;
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.BattleNode.Sessions;
+
+/// <summary>
+/// One side of a battle. Two of these are held by a <c>BattleSession</c>; the session
+/// brokers between them. Concrete impls (added in subsequent Phase-1 tasks):
+/// <list type="bullet">
+///   <item><c>RealParticipant</c> — WS-backed.</item>
+///   <item><c>NoOpBotParticipant</c> — silent; for <c>BattleType.Bot</c> (AI-passive).</item>
+///   <item><c>ScriptedBotParticipant</c> — wraps the v1.2 lifecycle for
+///         <c>BattleType.Scripted</c> (solo testing harness).</item>
+/// </list>
+/// </summary>
+public interface IBattleParticipant : IAsyncDisposable
+{
+    /// <summary>Real viewer id, or a synthetic stable id for bots
+    /// (<see cref="Lifecycle.ScriptedLifecycle.FakeOpponentViewerId"/>).</summary>
+    long ViewerId { get; }
+
+    /// <summary>Per-battle MatchContext snapshot, used for building Matched/BattleStart
+    /// selfInfo when this participant is "self" in the perspective.</summary>
+    MatchContext Context { get; }
+
+    /// <summary>Session calls this to deliver a frame from the OTHER participant
+    /// (or a server-synthesized broadcast). Real impl: encode + WS-send.
+    /// NoOp: swallow. Scripted: may emit a response via <see cref="FrameEmitted"/>.</summary>
+    /// <param name="noStock">True for control frames (BattleFinish, JudgeResult, ack);
+    /// bypasses playSeq assignment + archive.</param>
+    Task PushAsync(MsgEnvelope envelope, bool noStock, CancellationToken ct);
+
+    /// <summary>Participant fires this when it has a frame to send TO the session
+    /// (its own gameplay action). Real impl: fires on WS recv. NoOp: never fires.
+    /// Scripted: fires from inside PushAsync when the scripted lifecycle wants to
+    /// respond to an inbound frame.</summary>
+    event Func<MsgEnvelope, CancellationToken, Task>? FrameEmitted;
+
+    /// <summary>Drives the participant's inbound loop. For Real: the WS read loop
+    /// (returns when the WS closes). For NoOp/Scripted: completes immediately (the
+    /// session keeps running as long as the OTHER participant's RunAsync is alive).</summary>
+    Task RunAsync(CancellationToken ct);
+
+    /// <summary>Called when the battle ends. Concrete impls clean up (close WS, etc.).</summary>
+    Task TerminateAsync(BattleFinishReason reason);
+}