`mesa-2.x` legacy branch for Mesa 2.x examples

We've created a [`mesa-2.x`](https://github.com/projectmesa/mesa-examples/tree/mesa-2.x) branch for Mesa 2.x examples, while we continue development towards Mesa 3.0.

For new pull requests:
- To update/add an example for Mesa 3.x, target the [`main`](https://github.com/projectmesa/mesa-examples/tree/main) branch (default)
- To update/add an example for Mesa 2.x, target the [`mesa-2.x`](https://github.com/projectmesa/mesa-examples/tree/mesa-2.x) branch (default)

We very much welcome new Mesa 3.0 features to be added to the examples, which should now be easier targeting only Mesa 3.0. See:
- https://github.com/projectmesa/mesa-examples/issues/111
- https://github.com/projectmesa/mesa-examples/issues/123
- https://github.com/projectmesa/mesa-examples/issues/124

This issue is a branch-management announcement rather than a scoped code task, and it names no files or tests. Review the mesa-2.x and main branches plus issues 111, 123, and 124 to understand the referenced Mesa 3.0 work. The payload does not define a concrete completion condition.

Written by the indexing model from the issue text.