feat(vigilante): add arena boss succession

design/user-stories/us-008-support-vigilante-and-followers.md

@@ -11,7 +11,7 @@ As the **Vigilante**, I want nearby living Followers to strengthen me against th
 
 ## Acceptance criteria
 
-- [x] Exactly one active Vigilante exists while the game is running whenever an eligible player is available.
+- [x] A running game has at most one active Vigilante; a vacancy remains open until an eligible player defeats the arena boss.
 - [x] The Vigilante can invite eligible opted-in players to become Followers, and recruitment requires acceptance.
 - [x] The Vigilante can dismiss Followers and Followers can leave voluntarily.
 - [x] While actively fighting the Tyrant, the Vigilante gains one potion level each of Strength and Resistance per living Follower in the same world within 50 blocks.
@@ -19,17 +19,18 @@ As the **Vigilante**, I want nearby living Followers to strengthen me against th
 - [x] A Vigilante or Follower may simultaneously hold one Tyrant-side class without losing Vigilante-side membership.
 - [x] Effect levels update promptly as combat starts or ends and as Followers move, die, respawn, disconnect, or change assignment.
 - [x] Scaling is capped by validated configuration, defaulting to Strength V and Resistance IV.
-- [x] Every Vigilante death ends the assignment and triggers random replacement regardless of cause.
-- [x] The deceased Vigilante is excluded when another eligible non-Tyrant candidate exists and is reused only when no such candidate exists.
+- [x] Every Vigilante death ends the assignment and opens the one-player arena challenge while a Tyrant is active.
+- [x] The deceased Vigilante may immediately compete again under the same arena eligibility and reset rules as every other participant.
 - [x] Followers are cleared rather than transferred when their Vigilante dies or loses the position.
 - [x] The Tyrant is never eligible to be selected as Vigilante.
 - [x] The Vigilante can review, invite, and dismiss Followers through the Vigilante control panel.
 - [x] Control-panel and command-based Follower actions enforce identical eligibility and membership rules.
 - [x] The control panel communicates the current nearby-Follower combat contribution and configured effect caps.
-- [x] Relinquishing through the control panel follows normal Vigilante succession and Follower cleanup.
+- [x] Relinquishing through the control panel clears Followers and opens the Vigilante arena challenge.
 
 ## Related
 
 - [Role and class model](../concepts/role-and-class-model.md)
 - [Handle inactivity and pending selections](us-010-handle-inactivity-and-selection.md)
 - [Manage Followers through the Vigilante control panel](us-016-manage-vigilante-control-panel.md)
+- [Claim the Vigilante role in the arena](us-019-claim-vigilante-in-arena.md)