Add or remove an actor from a user's representing list, enforcing the
one-user-per-actor constraint.
When adding: the actor is removed from every other user's representing list
before being added to this one. Removing from other users' lists requires
GM permission; if the caller lacks it those removals are silently skipped
(the caller's own entry still gets set correctly).
When removing: only the target user's list is modified.
Parameters
user: User
actor: Actor
Optionalforce: boolean
If provided, sets the state explicitly instead of toggling.
Add or remove an actor from a user's representing list, enforcing the one-user-per-actor constraint.
When adding: the actor is removed from every other user's representing list before being added to this one. Removing from other users' lists requires GM permission; if the caller lacks it those removals are silently skipped (the caller's own entry still gets set correctly). When removing: only the target user's list is modified.