ETB doubling effects (e.g. Panharmonicon) do not apply to ZONE_CHANGE_GROUP events

Title says it all. Someone said it might be because his ability has a new keyword. Either way i m pretty sure i didnt miss anything, and it just did not trigger twice when a token entered the bf.

Start by tracing handling for ZONE_CHANGE_GROUP events and ETB doubling effects, reproducing the issue with a token entering the battlefield while Panharmonicon is present. Done means the relevant ETB effect is applied twice in that scenario, with regression coverage for the event path.

Written by the indexing model from the issue text.