lift main.rs business logic into the library

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.

src/package_info.rs

@@ -9,6 +9,22 @@ use crate::apt::release::{self, VerifiedRelease};
 use crossterm::style::Stylize;
 use log::{debug, warn};
 
+/// Split a PPA reference into its `(user, name)` parts
+///
+/// A PPA is written `user/ppa_name` (e.g. `user/my-ppa`); anything else —
+/// more segments, empty parts — is a format error carrying the canonical
+/// message.
+pub fn split_ppa(ppa: &str) -> Result<(&str, &str), String> {
+    let parts: Vec<&str> = ppa.split('/').collect();
+    if parts.len() == 2 && !parts[0].is_empty() && !parts[1].is_empty() {
+        Ok((parts[0], parts[1]))
+    } else {
+        Err(format!(
+            "Invalid PPA format: '{ppa}'. Expected: user/ppa_name"
+        ))
+    }
+}
+
 /// Convert a PPA specification to a base URL
 ///
 /// # Arguments
@@ -943,6 +959,25 @@ pub async fn lookup(
 mod tests {
     use super::*;
 
+    /// `user/ppa_name` splits into its two parts.
+    #[test]
+    fn split_ppa_parses_the_canonical_form() {
+        assert_eq!(split_ppa("user/my-ppa"), Ok(("user", "my-ppa")));
+    }
+
+    /// Anything but exactly two non-empty segments is rejected, with the
+    /// canonical message.
+    #[test]
+    fn split_ppa_rejects_malformed_references() {
+        for bad in ["", "user", "user/", "/ppa", "a/b/c"] {
+            let err = split_ppa(bad).unwrap_err();
+            assert!(err.contains("Invalid PPA format"), "{err}");
+            assert!(err.contains(bad), "{err}");
+        }
+    }
+
+    use super::*;
+
     /// Serve canned byte responses on a local port, one per connection (the
     /// last response repeats), and return the base URL
     ///