feat(tyrant): add arena boss succession

design/user-stories/us-002-transfer-and-reset-tyrant.md

@@ -12,11 +12,11 @@ As a **player**, I want defeating the Tyrant to transfer power predictably so th
 ## Acceptance criteria
 
 - [x] A player directly credited with killing the Tyrant becomes the new Tyrant.
-- [x] A Tyrant death without a player killer selects an eligible opted-in player active within the preceding 24 hours at random.
-- [x] The former Tyrant is excluded from random succession when another eligible candidate exists.
+- [x] A Tyrant death without a player killer leaves the role vacant and opens the shared Tyrant arena challenge.
+- [x] The former Tyrant may compete again under the same arena eligibility and reset rules as every other opted-in participant.
 - [x] The former Tyrant's level, unlocks, unspent choices, and Tyrant ability cooldowns reset when the reign ends.
 - [x] Every Assassin, Fixer, Tamer, Vigilante, and Follower assignment is cleared when the Tyrant dies.
-- [x] A new Vigilante is selected under the normal candidate rules after succession.
+- [x] After Tyrant succession, a vacant Vigilante role opens the shared arena's Vigilante challenge.
 - [x] The new Tyrant receives exactly one starting unlock choice.
 - [x] The Tyrant's displayed name is red and the Tyrant has a red glowing outline while the game is running.
 - [x] Succession cannot assign the same player as both Tyrant and Vigilante.
@@ -25,3 +25,4 @@ As a **player**, I want defeating the Tyrant to transfer power predictably so th
 
 - [Tyrant game state machine](../concepts/game-state-machine.md)
 - [Progress and purchase Tyrant unlocks](us-003-progress-and-purchase-unlocks.md)
+- [Claim the Tyrant role in the arena](us-020-claim-tyrant-in-arena.md)