V4.8.1 MULTI-ENTRANCE FIX

AREA TRANSITIONS / ENTRANCES:
- CREATE ARRIVAL FOR THIS INCOMING EXIT is now strictly add-only.
- Creating an arrival for another incoming exit never deletes or overwrites previous entrances.
- Each incoming source exit receives a unique route-specific entrance name.
- Only explicitly selecting an existing entrance allows that entrance to be updated.
- LINK SELECTED ENTRANCE TO INCOMING EXIT remains available when routes should intentionally share an entrance.
