V252 - NORMAL ACCOUNT IMPORT/EXPORT + JOIN/LEAVE ARBITRATION

MULTIPLAYER
- Arbitration is requested only by a real remote-player join edge (empty slot -> named player) or leave edge (named player -> empty slot).
- Pressing Play arms a transition guard before BG2's native Play action. Clearing or refreshing multiplayer rows during the transition does not open Arbitration.
- Kick cleanup and disconnect removal do not copy, back up, restore, or respawn character files.

CHARACTER FOLDERS
- A real/shared `characters` directory is denied and deleted if it appears.
- `characters_<account>` is allowed only when its full account suffix and active account flag match the logged-in account. Other account folders are denied.
- After login, BG2's required engine-facing `characters` path is a junction to only the matching `characters_<account>` folder. This lets BG2's stock Import and Export screens work directly without a blocking preparation panel. The junction is detached on logout, quit, or router shutdown.
- Import and Export rows are refreshed and filtered to the active account. Native Done, Cancel, filename entry, create-new, and overwrite behavior stays available; all writes go through the exact account junction.
- CHR/BIO files are not copied or backed up. DELETE ACCOUNT remains an explicit, password-confirmed permanent deletion of that account folder.

VALIDATION
- Regression coverage checks exact account-name matching, deletion of a real shared folder, exact-account session junction lifecycle, native Import/Export callbacks without the preparation modal, and join/leave-only Arbitration triggers.
- Run `RUN_IN_GAME_ACCOUNT_TESTS.bat` on Windows for the focused package checks.
- UI.MENU structural validation and account/Lua regressions run in the included Python tests.
- The modified BG2 engine UI was not launched in a Windows client here; confirm the final callback feel in-game.
