build: add differential tests against real dpkg-buildpackage

Add an automated differential test harness in build/mod.rs that builds
the same source tree twice - once with real 'dpkg-buildpackage -S
-I -i -nc -d --no-sign', once with the native pipeline - and compares
all produced artifacts:

- .dsc payload byte-for-byte,
- .changes field-by-field (checksum lines of the .buildinfo itself
  excluded, as its content legitimately differs on machine-dependent
  fields),
- .buildinfo structure (Installed-Build-Depends, Environment,
  Build-Date and Build-Tainted-By excluded).

src/build/changes.rs

@@ -34,6 +34,8 @@ pub struct ChangesInput {
     pub changed_by: Option<String>,
     /// Formatted per-package description lines (empty for source-only).
     pub descriptions: Vec<String>,
+    /// Bug numbers collected from the changelog (`Closes` field), if any.
+    pub closes: Option<String>,
     /// Rendered `Changes` field value from the changelog entry.
     pub changes_field: String,
     /// Computed artifact checksums (dsc, tarballs, debs, buildinfo).
@@ -111,6 +113,9 @@ pub fn render_changes(input: &ChangesInput) -> Paragraph {
         // Leading `\n`: pre-wrapped multiline field, dpkg-style.
         p.set("Description", &format!("\n{}", sorted.join("\n")));
     }
+    if let Some(closes) = &input.closes {
+        p.set("Closes", closes);
+    }
     p.set("Changes", &input.changes_field);
 
     if !input.checksums.is_empty() {
@@ -199,6 +204,7 @@ mod tests {
             maintainer: Some("A B <a@b.c>".to_string()),
             changed_by: Some("A B <a@b.c>".to_string()),
             descriptions: vec![],
+            closes: None,
             changes_field: "pkg (1.0) unstable; urgency=medium\n.\n * Something.".to_string(),
             checksums,
             files_list,