Function scaffold_plan 

pub fn scaffold_plan() -> Vec<FileSpec>

Compute the set of files a fresh course scaffold writes.

Pure (§2.1, §2.2): it returns the plan as data with no filesystem access, so the file set — and that each template is internally valid — is asserted directly in a unit test. scaffold_course is the effectful step that writes it (§5.1, plan vs write split).