feat(vigilante): add arena boss succession

design/concepts/game-state-machine.md

@@ -16,20 +16,23 @@ Pausing freezes gameplay progression, role inactivity checks, pending-selection
 
 When a player directly kills the Tyrant, that killer becomes the new Tyrant. When no player is credited with the death, an eligible opted-in player active during the preceding 24 hours is selected randomly. The former Tyrant is excluded when another candidate exists.
 
-Every Tyrant death ends the reign, clears all role-side assignments, resets the former Tyrant's reign progression, and requires a new Vigilante. The new Tyrant receives one starting unlock choice.
+Every Tyrant death ends the reign, clears all role-side assignments, resets the former Tyrant's reign progression, and leaves the Vigilante role open for the arena challenge. The new Tyrant receives one starting unlock choice.
 
 ## Vigilante succession
 
-Every Vigilante death ends that assignment regardless of cause. A replacement is selected randomly from eligible opted-in players active during the preceding 24 hours. The Tyrant is never eligible. The previous Vigilante is excluded when another eligible non-Tyrant exists and may be selected again only as a fallback. Followers are cleared rather than transferred.
+Every Vigilante death ends that assignment regardless of cause. Vigilante inactivity and relinquishment also end the assignment, and every new Tyrant reign clears the previous assignment. Followers are cleared rather than transferred.
 
-A Tyrant earns progression from the death only when the Tyrant personally killed the Vigilante.
+While the game is running with an active Tyrant and no Vigilante, an administrator-configured arena opens. One eligible opted-in non-Tyrant player at a time may challenge the arena boss. Defeating it assigns that player as Vigilante. Leaving, disconnecting, or dying resets the fight without preventing an immediate retry. Without an active Tyrant or configured arena, the role remains vacant and no boss exists.
+
+A Tyrant earns progression from a Vigilante death only when the Tyrant personally killed the Vigilante.
 
 ## Pending selections
 
-An offline random selectee has 24 hours of unpaused time to log in and confirm the assignment. Failure causes another selection. When no eligible candidate exists, the position remains vacant and selection retries periodically without assigning the Tyrant to both roles.
+An offline randomly selected Tyrant has 24 hours of unpaused time to log in and confirm the assignment. Failure causes another Tyrant selection. When no eligible Tyrant candidate exists, the position remains vacant and selection retries periodically. Vigilante vacancies use the arena rather than pending random selection.
 
 ## Related stories
 
 - [Start, pause, and administer the game](../user-stories/us-001-start-pause-and-administer.md)
 - [Transfer and reset the Tyrant role](../user-stories/us-002-transfer-and-reset-tyrant.md)
 - [Handle inactivity and pending selections](../user-stories/us-010-handle-inactivity-and-selection.md)
+- [Claim the Vigilante role in the arena](../user-stories/us-019-claim-vigilante-in-arena.md)