resolveCfgFile
fun resolveCfgFile(root: File, at: AnalysisType, depth: AnalysisDepth, mode: MergeMode, slug: String): File
Resolves cfgRelPath against root and ensures the parent directory exists on disk. Centralises the "compose path + mkdirs" boilerplate that producers (run-plan generator) repeat at every cfg-write site.
The cfg file itself isn't created — only its parent directory.