V257 - PLAYER LEAVE REMOVES LIVE CHARACTER THROUGH CHARACTER ARBITRATION
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Player leave
------------
- A real remote player 2-6 leaving creates one disconnect cleanup event.
- The host remembers the departed player's bound character slot before clearing
  the player's network/account mapping.
- The live party character is NOT deleted from the world screen.
- The genuine Player-1 host opens Character Arbitration through the same left
  sidebar Character Arbitration button behavior used by BG2.
- Once Character Arbitration is actually active, V257 selects the departed slot
  and calls BG2's native OnModifyCharacterDelete() path.
- The slot/account binding is released only after BG2 reports the live character
  as removed.
- The permanent account CHR/BIO file is not deleted, copied, restored, or backed up.

Arbitration
-----------
- Only BG2's genuine multiplayer host running as local Player 1 may force the
  Character Arbitration transition or remove a departed live party character.
- If another modal is active when the player leaves, the cleanup stays pending.
  It opens Character Arbitration and applies the removal when the world screen is
  available again.
- The leave event is one-shot. After cleanup, Play returns to gameplay normally
  and the same leave event does not reopen Arbitration.
- Clients 2-6 cannot invoke host Arbitration controls or delete party characters.

Export
------
- V257 no longer uses a departed-account Export lock as a substitute for party
  cleanup. Normal account-bound Export rules remain unchanged.

Join protection
---------------
- V254's one-shot Join protection remains active.
- Already-connected clients cannot resend a Direct-IP/session Join request from gameplay.

Auto-Save
---------
- V256's SAVENAME.2DA change is preserved: standard Auto-Save and Auto-Save-TOB
  entries remain disabled.
- Existing non-auto save rows are preserved and no .bak file is created.

Testing
-------
- RUN_IN_GAME_ACCOUNT_TESTS.bat runs the V257 suite.
- Tests cover immediate and deferred leave cleanup, native Arbitration deletion,
  host-only authority, normal Play return, no Export-lock dependency, one-shot
  Join behavior, native Import/Export regressions, and SAVENAME.2DA handling.
