The chlog target-series resolution becomes changelog::series_candidates
(UNRELEASED pinning, development-series default and fallbacks modeled
by SeriesCandidates), PPA references get package_info::split_ppa
(shared by pull and deb, now also rejecting empty parts), and SSH
endpoints get context::ContextConfig::from_endpoint — so a library
consumer can resolve series, validate PPAs and build context
configurations without reimplementing the CLI's rules. All three carry
unit tests; main.rs keeps only parsing of flags and error handling.