feat(CS-P3): throne registration via Sinior/Crown talk (223/216) after defenses down + both switches held; reachable defense positions

src/GameLogic/CastleSiege/CastleSiegeContext.cs

@@ -169,26 +169,38 @@ public class CastleSiegeContext
     }
 
     /// <summary>
-    /// Evaluates the throne capture: if the siege is running, all defenses are destroyed, the throne is not
-    /// yet taken, and one guild holds BOTH Crown Switches at once, that guild captures the throne.
+    /// Attempts to capture the throne for a guild (called when a member registers at the Sinior/Crown NPC).
+    /// Succeeds only during the siege when the throne is free, all castle defenses are destroyed, and the
+    /// guild is currently holding BOTH Crown Switches (a member standing on each).
     /// </summary>
-    /// <returns>The guild that just captured the throne (for announcing), or <c>null</c> if nothing changed.</returns>
-    public string? EvaluateCapture()
+    /// <param name="guildName">The capturing guild's name.</param>
+    /// <returns>Whether it succeeded and a human-readable reason/result message.</returns>
+    public (bool Success, string Reason) TryCaptureThrone(string guildName)
     {
-        if (this.Phase != CastleSiegePhase.Siege || this._occupier is not null || this._defensesRemaining > 0)
+        if (this.Phase != CastleSiegePhase.Siege)
         {
-            return null;
+            return (false, "The siege is not running.");
         }
 
-        var holder217 = this._switchHolders[217];
-        var holder218 = this._switchHolders[218];
-        if (holder217 is not null && holder217 == holder218)
+        if (this._occupier is not null)
         {
-            this._occupier = holder217;
-            return holder217;
+            return (false, this._occupier == guildName
+                ? "Your guild already holds the throne."
+                : $"The throne is already held by '{this._occupier}'.");
         }
 
-        return null;
+        if (this._defensesRemaining > 0)
+        {
+            return (false, $"Destroy the castle defenses first ({this._defensesRemaining} remaining).");
+        }
+
+        if (this._switchHolders[217] != guildName || this._switchHolders[218] != guildName)
+        {
+            return (false, "Your guild must be holding BOTH Crown Switches at once (stand a member on each).");
+        }
+
+        this._occupier = guildName;
+        return (true, "throne captured");
     }
 
     /// <summary>Returns a human-readable status summary for admin display.</summary>