Re-design DSA migrator #5441
Description
Earlier in 2025, the team worked on a DSA migrator to handle the one-time migration of digital serials metadata from Cocina descriptive to identification metadata, and the process of writing and documenting the migrators and trying to write reliable, high-quality tests raised questions about the overall architecture, how long & where the migrator code should live, and how best to test migrators. We should consider re-designing the migrator so that less code lives in an untestable script and there are fewer surprises. Ideally we would do this work as part of a work cycle that needs to use or make changes to the DSA migrator framework.