fix(arena): improve containment and boss idle

design/user-stories/us-019-claim-vigilante-in-arena.md

@@ -17,11 +17,14 @@ As an **opted-in participant**, I want a visible one-player boss challenge for a
 - [x] The challenge boss defaults to an armored, 80-health enhanced zombie, does not burn in sunlight, and remains within a default 10-block-radius arena.
 - [x] All online opted-in players are told when the role opens and receive the arena world and coordinates; opted-in players receive the same notice when they log in while it remains open, and opted-out players do not.
 - [x] Exactly one eligible opted-in non-Tyrant player may enter as challenger, including when entry is attempted by walking or teleportation.
-- [x] Starting a fight replaces the passive particle boundary with temporary actual `BARRIER` blocks that are removed safely without permanently changing existing blocks.
+- [x] Starting a fight builds temporary actual `BARRIER` blocks from one block below the arena floor through a default 12-block wall height, without permanently changing existing blocks.
+- [x] Role-colored boundary particles remain visible from inside and outside an active fight at multiple wall heights.
 - [x] Any non-challenger who enters or respawns inside the arena is moved safely outside it.
 - [x] Existing mobs in the arena and mobs that spawn or enter it are removed while preserving the arena boss.
+- [x] With no challenger, the boss remains AI-disabled at the arena center; entry enables its AI and target, while reset restores full health, center position, no target, and disabled AI.
 - [x] The boss attacks only the active challenger, and only that challenger can damage the boss.
 - [x] Challenger death, disconnect, or leaving by any mechanism ends the fight and resets the boss to full health at its spawn; death counts as leaving and the same player may retry immediately.
+- [x] Block placement and breaking are denied inside the arena during an active fight so players cannot build over or modify the boundary.
 - [x] Killing the boss assigns the challenger as Vigilante, removes the boss and barriers, and restores the passive particle boundary.
 - [x] Pausing, resetting, restarting, reloading, and repeated or concurrent events cannot duplicate the boss, barriers, challenger, or Vigilante assignment.
 - [x] If the arena is not configured when a challenge should open, the Vigilante remains vacant and administrators receive a clear warning.