Build a periodic direct/indirect, CPU/GPU problem bank (post v2.1.0-beta)

Once the dependency upgrade in #826 (tracked from #815) is merged, we should build out a proper problem bank: a collection of optimal control problems solved through both the **direct** and **indirect** methods, on both **CPU** and **GPU**, run on a **periodic** basis (e.g. nightly or weekly CI) rather than only as part of the regular test suite.

The upgrade PR only wires up a handful of problems in dual `(:abstract, :functional)` form to unblock the migration — this issue is about growing that into a broader, recurring regression bank (many more problems, reference solutions, CPU/GPU parity checks) once things have settled.

No need to spec out the details here — this is a placeholder to come back to.