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 Filenames 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.