Deployment file semantics

A few topics that came up internally, that could be good to clarify

- Will it be possible to have multiple deployment files, e.g. file `a.yml` adding key `a` to namespace `seats`, then file `b.yml` adding yet another key `b` and `c` to namespace `seats`?
- If so then tooling must consider order to correctly handle overrides, right?
- Will it be possible to use complex values (e.g. maps) for keys in deployment files?

No implementation file or test is named. Start with the repository README and linked project documentation to determine the intended deployment-file semantics, including multiple files, ordering, overrides, and map values. Done means recording an explicit decision and identifying the affected tooling or documentation.

Written by the indexing model from the issue text.