pet groups should naturally be dissolved after a certain age

**context:** whenever a new group type is introduced to the game, it's hard for old pets to get involved, since their groups are probably stable.

it's also just nice to have pets do new things with new pets!

**todo:** after a certain group age, have an increasing chance that the pets in the group mutually decide to move on to other things, and dissolve the group.

the group should conclude with a final project that's unique from the other projects groups complete.

The issue does not identify any files, tests, or entry points to read or run. Before implementation, clarify the age threshold, increasing probability, dissolution behavior, and how the unique final project differs from existing group projects; done means these rules are implemented and verified.

Written by the indexing model from the issue text.