V251 - JOIN/LEAVE ARBITRATION + EXACT ACCOUNT-FOLDER ACCESS

MULTIPLAYER
- The Arbitration screen is queued 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. It stays
  armed through the initial world-screen refresh so BG2 clearing its multiplayer
  rows cannot reopen Arbitration. A timeout clears the guard if Play is cancelled.
- Kick cleanup and disconnected-character removal do not copy, back up, restore,
  or respawn character files.

CHARACTER FOLDERS
- `characters` is denied and remains absent outside native Import/Export.
- If a real directory appears at the denied `characters` path, the router deletes
  it on the next check. This is the requested shared-folder deletion behavior;
  no backup or parked copy is created.
- `characters_<account>` is allowed only when the full folder suffix and active
  account flag match the logged-in account. All other account folders are denied.
- BG2's stock Import/Export needs a path literally named `characters`. V251 mounts
  only the matching account folder for the active native panel and detaches it on
  Done/Cancel. Import and Export lists are filtered to that matching account.
- CHR/BIO files are not copied or backed up. DELETE ACCOUNT remains an explicit,
  password-confirmed permanent deletion of that account's folder.

VALIDATION
- Regression coverage checks exact account-name matching, login denial of the
  plain folder, temporary Import/Export mount and close, shared-folder deletion,
  and join/leave-only Arbitration triggers.
- 52 multiplayer/account regressions passed; all 17 account/Lua/UI unit tests
  passed; Python compilation and the installer smoke test passed.
- UI.MENU structural validation found 138 named menus with no structure errors.
- BG2 was not launched in a Windows client here; the final engine callbacks and
  Windows junction lifecycle still need an in-game check.
