feat(battle-node): add NoOpBotParticipant

Silent participant for the Phase 3 Bot type. PushAsync swallows;
FrameEmitted never fires; RunAsync completes immediately. ViewerId is
the existing FakeOpponentViewerId const for consistency with scripted
lifecycle builders. Three tests lock the no-op contract.

SVSim.BattleNode/Sessions/Participants/NoOpBotParticipant.cs

@@ -0,0 +1,35 @@
+using SVSim.BattleNode.Bridge;
+using SVSim.BattleNode.Lifecycle;
+using SVSim.BattleNode.Protocol;
+
+namespace SVSim.BattleNode.Sessions.Participants;
+
+/// <summary>
+/// Silent participant — produces no frames, swallows everything pushed to it.
+/// Used as the "other" participant in <see cref="BattleType.Bot"/> sessions, where
+/// the real opponent runs in the client and the server has no opponent-side state
+/// to model. ViewerId is <see cref="ScriptedLifecycle.FakeOpponentViewerId"/>;
+/// Context is a fixed stub (irrelevant — never read because no frames are pushed
+/// to the other side).
+/// </summary>
+public sealed class NoOpBotParticipant : IBattleParticipant
+{
+    public long ViewerId => ScriptedLifecycle.FakeOpponentViewerId;
+    public MatchContext Context { get; } = new(
+        SelfDeckCardIds: Array.Empty<long>(),
+        ClassId: "0", CharaId: "0", CardMasterName: "card_master_node_10015",
+        CountryCode: "", UserName: "Bot", SleeveId: "0",
+        EmblemId: "0", DegreeId: "0", FieldId: 0, IsOfficial: 0,
+        BattleType: 0);
+
+    public event Func<MsgEnvelope, CancellationToken, Task>? FrameEmitted;
+
+    public Task PushAsync(MsgEnvelope envelope, bool noStock, CancellationToken ct) => Task.CompletedTask;
+    public Task RunAsync(CancellationToken ct) => Task.CompletedTask;
+    public Task TerminateAsync(BattleFinishReason reason) => Task.CompletedTask;
+    public ValueTask DisposeAsync() => ValueTask.CompletedTask;
+
+    // Suppress unused-event warning — FrameEmitted is declared by the interface contract;
+    // intentionally never invoked.
+    private void Touch() => FrameEmitted?.Invoke(null!, default);
+}