V243 - ARBITRATION ONLY ON ACTUAL PLAYER LEAVE

- Keeps V242 one-account-per-running-game rule unchanged.
- Keeps account-bound slots/characters, strict account-folder router law, localhost Join default, and NO BACKUPS behavior.
- Multiplayer binding/refresh pulses can no longer trigger a leave event.
- Arbitration is forced only when BG2 reports a real connected player-name transition from non-empty to empty.
- Normal joining, player-row refreshes, account binding, and temporary blank rows do not open Arbitration.
- When a player really leaves, the host returns to Arbitration and the departed live character is removed there while the account/slot reservation remains.
- Installer runtime validation now checks the current V243 runtime instead of the obsolete V206 marker.
