deb,build: export ddebs from binary builds

dh_strip registers debug-symbol packages in debian/files under their
renamed .ddeb name; they are deliverables of the build just like the
regular .debs. Both artifact-type filters, the one collecting the
build's artifacts and the one feeding the .buildinfo/.changes
checksum sections, accepted only deb and udeb, so every ddeb a build
produced was silently discarded: nothing exported next to the .debs,
nothing in the upload metadata.

The filter is now a shared FilesEntry::is_binary_artifact() predicate
(plus unit tests) used by both call sites, so the two cannot drift
apart again.

One dpkg distinction has to be copied rather than shared:
dpkg-genchanges lists only deb/udeb packages in the changes' Binary:
and Description (a ddeb travels in Files and the checksum sections
but is not a listed binary of the upload), while dpkg-genbuildinfo
lists every binary artifact's package, ddebs included. The package
list is therefore accumulated twice, once per consumer.

The binary-metadata differential fixture gains a Package-Type: ddeb
package, so the native .changes/.buildinfo generation is compared
against real dpkg with a ddeb present. Without the fix the comparison
fails on the checksum sections (golden distributes the ddeb, ours
dropped it); with it every field matches.

src/debian/files.rs

@@ -46,6 +46,21 @@ impl FilesEntry {
         entry.priority = priority.to_string();
         entry
     }
+
+    /// Whether this entry is a binary-package artifact that must be
+    /// exported from the build: a `.deb`, a `.udeb`, or a `.ddeb`.
+    ///
+    /// Debug-symbol packages are registered in `debian/files` by
+    /// `dh_strip` under their renamed `.ddeb` name; they are as much
+    /// a deliverable of the build as the regular `.deb`s (they belong
+    /// in the upload set), so any filter keyed on artifact type has
+    /// to accept them.
+    pub fn is_binary_artifact(&self) -> bool {
+        matches!(
+            self.package_type.as_deref(),
+            Some("deb") | Some("udeb") | Some("ddeb")
+        )
+    }
 }
 
 /// Derive metadata from an artifact file name following the common
@@ -243,6 +258,21 @@ mod tests {
         assert_eq!(e.package_type, None);
     }
 
+    #[test]
+    fn binary_artifact_filter_accepts_ddebs() {
+        let deb = parse_filename("hello_2.10-3_amd64.deb").unwrap();
+        let udeb = parse_filename("hello_2.10-3_amd64.udeb").unwrap();
+        let ddeb = parse_filename("hello-dbgsym_2.10-3_amd64.ddeb").unwrap();
+        let buildinfo = parse_filename("hello_2.10-3_amd64.buildinfo").unwrap();
+        let changes = parse_filename("hello_2.10-3_amd64.changes").unwrap();
+
+        assert!(deb.is_binary_artifact());
+        assert!(udeb.is_binary_artifact());
+        assert!(ddeb.is_binary_artifact(), "ddebs must be exported");
+        assert!(!buildinfo.is_binary_artifact());
+        assert!(!changes.is_binary_artifact());
+    }
+
     #[test]
     fn load_save_roundtrip() {
         let dir = tempfile::tempdir().unwrap();