build: stop redistributing the previous source on binNMU uploads

dpkg-genchanges/genbuildinfo handle a binary-only upload by referencing
the previous source version textually (Source: pkg (prev),
Binary-Only: yes, Binary-Only-Changes) while distributing no source
files at all: pkh instead pulled the previous .dsc and its tarballs
into both documents whenever they sat next to the artifacts, re-uploading
the whole source on every binNMU.

Drop that redistribution (and include_dsc_artifacts with it), and emit
the missing Binary-Only: yes field, which the new differential test
against real dpkg-buildpackage -b caught. The binNMU case shares its
runner with the regular binary metadata differential; a unit test pins
the exclusion even with the previous artifacts present.

src/build/changes.rs

@@ -65,6 +65,9 @@ pub struct ChangesInput {
     pub source: String,
     /// Sorted binary package names with artifacts (empty for source-only).
     pub binaries: Vec<String>,
+    /// Whether the changelog entry is a binary-only (binNMU) upload
+    /// (`Binary-Only: yes` field).
+    pub binary_only: bool,
     /// Active build profiles (`Built-For-Profiles`); omitted when empty.
     pub built_for_profiles: Vec<String>,
     /// `Architecture` field value in encounter order (e.g. `source`,
@@ -142,6 +145,9 @@ pub fn render_changes(input: &ChangesInput) -> Paragraph {
         let joined = input.binaries.join(" ");
         p.set("Binary", &wrap_long(&joined));
     }
+    if input.binary_only {
+        p.set("Binary-Only", "yes");
+    }
     if !input.built_for_profiles.is_empty() {
         p.set("Built-For-Profiles", &input.built_for_profiles.join(" "));
     }
@@ -329,6 +335,7 @@ mod tests {
             date: "Sat, 22 Aug 2026 10:00:00 +0000".to_string(),
             source: "pkg".to_string(),
             binaries: vec![],
+            binary_only: false,
             built_for_profiles: vec![],
             architecture: "source".to_string(),
             version: "1.0".to_string(),