# Issue

**Title:** Filename defaults and relative paths behave unexpectedly

## Description
Currently, `Filename.default` values are strictly validated as if they were final filenames, preventing the use of template-like default values. This restricts the flexibility of defining default filename patterns. Additionally, it's not possible to define a `Filename` that is relative to another `Filename` if the parent `Filename` is only defined via a default value and not explicitly present in the configuration.

The expected behavior is for `Filename.default` to accept any template-like string, and for relative `Filename` definitions to correctly resolve against parent `Filename`s that have default values, even if those parents are not explicitly configured.

## Task

Modify the repository so that the issue described above is resolved. The task's test suite verifies your patch by applying it on top of the base commit `72eb9a44f398` and running the modified tests.