fix(CS): stop crown re-capture loop + show seal panel only to the master

- The occupier can no longer re-register its own throne (canCapture = eligible != occupier),
  which caused a repeating 60s panel loop; only a DIFFERENT guild can contest.
- The 60s registration panel (0xB2/0x15) is now sent ONLY to the master on the crown,
  not broadcast to switch-holders/other players. Shield(0x16) + capture(0x18) still broadcast.
+1 test (15 total).

src/GameLogic/CastleSiege/CastleSiegeContext.cs

@@ -231,7 +231,11 @@ public class CastleSiegeContext
 
         var wasHolding = this._crownHoldGuild is not null;
 
-        if (eligibleGuild is null || !masterHolding)
+        // The guild that already occupies the throne just holds it — no re-registration (avoids a capture loop).
+        // Only a DIFFERENT guild taking both switches can register/capture (contest).
+        var canCapture = eligibleGuild is not null && eligibleGuild != this._occupier;
+
+        if (!canCapture || !masterHolding)
         {
             this._crownHoldGuild = null;
             this._crownHoldStartUtc = null;