V248 - SLOT STATE LAW + ACCOUNT EXISTENCE LAW

Multiplayer slot law:
- while a slot has no connected player owner, it is unselectable
- an empty/unowned slot clears any stale live/selected character
- an empty/unowned slot is unaccountbound
- while a connected player fills a slot, that slot is accountbound to that player's account
- leave/kick removes the live character and releases the slot binding
- no CHR/BIO account file is deleted and no backup/restore behavior is added

Account login law:
- successful login sets ACCOUNT Existence=1
- while ACCOUNT Existence=1, another login for the same account is denied
- one running BG2 game still permits only one logged-in account
- Exit, Quit, main-menu Exit, and confirmed window-X set that account Existence=0
- legacy Activate/Disabled flags remain synchronized only for compatibility
