V250 - PLAY TRANSITION GUARD + EXACT ACCOUNT-FOLDER MATCH

MULTIPLAYER
- Pressing Play/Done clears the pending Arbitration state and arms a transition
  guard before calling BG2's native multiplayer Done action.
- Native player-row clears/rebuilds during campaign startup still perform slot
  and live-character cleanup, but do not count as a new disconnect that reopens
  Arbitration.
- The transition guard clears when the world screen is active. A later real
  remote-player join or leave still requests one Arbitration visit.
- Kicked/disconnected live characters are removed from the session. Account
  character files are not backed up, restored, or deleted by this fix.

ACCOUNT CHARACTER-FOLDER LAW
- The logged-in account name must match the complete characters_<accountname>
  folder suffix and that folder's stored account flag.
- The old shared characters directory is parked and is never an Import/Export
  source. A mismatched or inactive account folder fails closed.
- BG2's native Import/Export still requires a path literally named characters.
  While logged in, that engine path is a controlled junction to only the
  matching characters_<accountname> folder. It never points to the old shared
  folder or another account's folder.
- The router rechecks and reapplies this mapping every 100 ms. When no account is
  active, characters is absent and denied.

No CHR/BIO backup or automatic restoration is added.

VALIDATION
- 35 targeted regressions passed across the V240/V241/V243/V244/V247/V248/V249
  and V250 account, slot, import-path, and Arbitration checks.
- Python source compilation passed; UI.MENU validation found 138 named menus and
  no basic-structure errors.
- The package was not launched in a Windows BG2 client here, so the final
  multiplayer Play/join/leave behavior and Windows junction operation still need
  a live game test.
