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

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+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);
+}

SVSim.UnitTests/BattleNode/Sessions/Participants/NoOpBotParticipantTests.cs

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+using NUnit.Framework;
+using SVSim.BattleNode.Bridge;
+using SVSim.BattleNode.Protocol;
+using SVSim.BattleNode.Protocol.Bodies;
+using SVSim.BattleNode.Sessions;
+using SVSim.BattleNode.Sessions.Participants;
+
+namespace SVSim.UnitTests.BattleNode.Sessions.Participants;
+
+[TestFixture]
+public class NoOpBotParticipantTests
+{
+    [Test]
+    public void PushAsync_swallows_without_firing_FrameEmitted()
+    {
+        var p = new NoOpBotParticipant();
+        var fired = 0;
+        p.FrameEmitted += (_, _) => { fired++; return Task.CompletedTask; };
+
+        var env = new MsgEnvelope(
+            NetworkBattleUri.TurnEnd, ViewerId: 1, Uuid: "u", Bid: null, Try: 0,
+            Cat: EmitCategory.Battle, PubSeq: null, PlaySeq: null,
+            Body: new ResultCodeOnlyBody());
+
+        Assert.DoesNotThrowAsync(() => p.PushAsync(env, noStock: false, CancellationToken.None));
+        Assert.That(fired, Is.EqualTo(0));
+    }
+
+    [Test]
+    public async Task RunAsync_returns_immediately()
+    {
+        var p = new NoOpBotParticipant();
+        await p.RunAsync(CancellationToken.None);
+        // If we got here, it returned.
+        Assert.Pass();
+    }
+
+    [Test]
+    public void ViewerId_is_FakeOpponent()
+    {
+        var p = new NoOpBotParticipant();
+        Assert.That(p.ViewerId, Is.EqualTo(SVSim.BattleNode.Lifecycle.ScriptedLifecycle.FakeOpponentViewerId));
+    }
+}