feat(CS): on-map siege countdown timer (S6 client protocol C1 B2 17/1E)

Client already renders the siege-map countdown but never received the packets.
New ICastleSiegeStatusViewPlugIn + RemoteView impl send the raw S6 packets:
C1 B2 17 (battle start/stop flag, arms the countdown) and C1 B2 1E (remaining
hour/minute). CastleSiegeEventPlugIn broadcasts them to players on the battle map
every 10s during Siege and sends stop at Settlement. No client changes. +1 test.

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

tests/MUnique.OpenMU.Tests/CastleSiege/CastleSiegeContextTest.cs

@@ -179,6 +179,20 @@ public class CastleSiegeContextTest
         Assert.That(ctx.ConsumeDirty(), Is.True);
     }
 
+    /// <summary>Tests that the remaining siege time counts down during the siege and is zero otherwise.</summary>
+    [Test]
+    public async Task RemainingSiegeTimeReflectsSiegePhaseAsync()
+    {
+        var ctx = new CastleSiegeContext(Config()); // 10 minute siege duration
+        Assert.That(ctx.GetRemainingSiegeTime(T0), Is.EqualTo(TimeSpan.Zero), "no siege running -> zero");
+
+        await ctx.ForceStartRegistrationAsync(T0);
+        await ctx.ForcePhaseAsync(CastleSiegePhase.Siege, T0);
+
+        Assert.That(ctx.GetRemainingSiegeTime(T0.AddMinutes(3)), Is.EqualTo(TimeSpan.FromMinutes(7)));
+        Assert.That(ctx.GetRemainingSiegeTime(T0.AddMinutes(15)), Is.EqualTo(TimeSpan.Zero), "past the end -> clamped to zero");
+    }
+
     private static CastleSiegeConfiguration Config() => new()
     {
         RegistrationDuration = TimeSpan.FromMinutes(5),