V255 - FORCE HOST/PROTAGONIST ARBITRATION ON PLAYER LEAVE - NO BACKUPS

Behavior:
- A real remote player leave (players 2-6) forces exactly one Arbitration visit on the actual BG2 host/protagonist.
- The leave pulse is generated even if BG2 already erased the departed player's character-slot mapping before the final empty-name callback.
- Non-host clients never open Arbitration, including a client temporarily reporting local player ID 1.
- Pressing Play consumes the leave event and returns to gameplay without reopening Arbitration.
- Existing V254 one-shot Join guards remain in place, so connected clients do not resubmit Join and do not receive the server-occupied popup from this account UI logic.
- Native account-bound Import/Export behavior is preserved.
- No automatic character backup or restore behavior is added.
