feat(battle-node): Gungnir alive-body builders (scs/ocs ONLINE placeholders)

SVSim.BattleNode/Reliability/Gungnir.cs

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+namespace SVSim.BattleNode.Reliability;
+
+/// <summary>
+/// Body builders for the alive channel. The timer/loop that drives 5s emits lives on
+/// BattleSession; this class is just the pure body-shape factory.
+/// v1 always reports scs/ocs=ONLINE — real disconnect detection is deferred.
+/// </summary>
+public static class Gungnir
+{
+    public static readonly TimeSpan EmitInterval = TimeSpan.FromSeconds(5);
+
+    public static Dictionary<string, object?> BuildAliveEmitBody(InboundTracker tracker) => new()
+    {
+        ["currentSeq"] = tracker.HighWaterMark,
+        // actionSeq omitted in v1 — no turn-transition flag yet.
+    };
+
+    public static Dictionary<string, object?> BuildAlivePushBody() => new()
+    {
+        ["scs"] = "ONLINE",
+        ["ocs"] = "ONLINE",
+    };
+}

SVSim.UnitTests/BattleNode/Reliability/GungnirTests.cs

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+using NUnit.Framework;
+using SVSim.BattleNode.Reliability;
+
+namespace SVSim.UnitTests.BattleNode.Reliability;
+
+[TestFixture]
+public class GungnirTests
+{
+    [Test]
+    public void BuildAlivePush_AlwaysReturnsScsOnlineOcsOnline()
+    {
+        var body = Gungnir.BuildAlivePushBody();
+        Assert.That(body["scs"], Is.EqualTo("ONLINE"));
+        Assert.That(body["ocs"], Is.EqualTo("ONLINE"));
+    }
+
+    [Test]
+    public void BuildAliveEmit_CarriesCurrentSeqFromTracker()
+    {
+        var tracker = new InboundTracker();
+        tracker.Observe(7);
+
+        var body = Gungnir.BuildAliveEmitBody(tracker);
+
+        Assert.That(body["currentSeq"], Is.EqualTo(7L));
+        Assert.That(body.ContainsKey("actionSeq"), Is.False);  // omitted in v1
+    }
+}