V242 - ONE ACCOUNT LOGIN PER GAME ONLY

Only this account-session rule changes from V241:
- One running BG2 game instance can have exactly one logged-in account.
- After an account is logged into that game instance, a second account login is refused.
- The existing per-account active flag still prevents the same account from being used by another active game instance.
- Existing Exit / confirmed window-X Exit behavior releases the logged-in account as before.
- V241 Law<gamefolderRouterSystem/>, multiplayer behavior, localhost join, strict account character routing, and no-backups behavior are unchanged.
