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>

src/GameLogic/CastleSiege/CastleSiegeContext.cs

@@ -193,6 +193,22 @@ public class CastleSiegeContext
         return scheduleTime <= nowTime && nowTime <= windowEnd;
     }
 
+    /// <summary>
+    /// Returns how much time is left in the running siege battle (the on-map countdown value), or
+    /// <see cref="TimeSpan.Zero"/> when the siege is not currently running.
+    /// </summary>
+    /// <param name="nowUtc">The current UTC time.</param>
+    public TimeSpan GetRemainingSiegeTime(DateTime nowUtc)
+    {
+        if (this.Phase != CastleSiegePhase.Siege)
+        {
+            return TimeSpan.Zero;
+        }
+
+        var remaining = (this._phaseStartedUtc + this.Configuration.SiegeDuration) - nowUtc;
+        return remaining > TimeSpan.Zero ? remaining : TimeSpan.Zero;
+    }
+
     /// <summary>
     /// Returns whether the persistable state changed since the last call, resetting the flag.
     /// Called each tick by the plugin to decide whether to write state to the database.