Cards exiled with Brainstealer Dragon should go to unique exile zone

Cards exiled with [[Brainstealer Dragon]] can be cast by its controller for as long as the cards remain exiled. Currently, the exiled cards go to the generic exile zone. Instead, they should go to a specific "Brainstealer Dragon" exile zone so as to make it possible for players to tell which cards are available to themselves / their opponents.

Locate the Brainstealer Dragon card implementation and the code that assigns exiled cards to an exile zone. Reproduce the interaction in a game or relevant card-engine tests, then verify that cards exiled by Brainstealer Dragon are visibly separated from the generic exile zone and remain distinguishable to each player.

Written by the indexing model from the issue text.