build/binary: native .buildinfo/.changes for binary builds (pkh deb)

Extend the metadata writers to binary-only uploads and wire them into
the 'pkh deb' flow:

- build/binary.rs generates <pkg>_<ver>_<arch>.buildinfo/.changes
  through any Context: debian/files consumption, encounter-order
  Architecture accumulation (sorted in .buildinfo like dpkg-genbuildinfo),
  sorted Binary lists, dpkg-formatted Description lines with udeb
  suffixes, Installed-Build-Depends closure over the context status DB,
  and binNMU handling (Source: pkg (prev), Binary-Only-Changes, previous
  .dsc redistribution);
- artifact digests are computed inside the context via coreutils
  (md5sum/sha1sum/sha256sum/stat) so chrooted/remote trees work;
- deb/local.rs runs the generation after 'rules binary', exports
  SOURCE_DATE_EPOCH from the changelog (reproducibility), and resolves
  vendor/profiles inside the context; deb/mod.rs retrieves the new
  artifacts alongside the debs;
- reusable helpers added: FilesList::parse/render,
  parse_changelog_entry_from_str, parse_previous_version_from_str,
  installed_build_depends_from_content.

Differential gate: same tree built with real 'dpkg-buildpackage -b' and
with the pkh flow; .changes/.buildinfo compared field-by-field modulo
machine-dependent fields, artifact checksums included.

src/build/buildinfo.rs

@@ -28,13 +28,6 @@ struct StatusDb {
 }
 
 impl StatusDb {
-    /// Parse a dpkg status file (e.g. `/var/lib/dpkg/status`).
-    fn load(path: &Path) -> Result<StatusDb, Box<dyn std::error::Error>> {
-        let content = std::fs::read_to_string(path)
-            .map_err(|e| format!("cannot read status file '{}': {}", path.display(), e))?;
-        Ok(Self::from_str(&content))
-    }
-
     fn from_str(content: &str) -> StatusDb {
         let mut db = StatusDb::default();
         for para in parse_paragraphs(content) {
@@ -132,7 +125,20 @@ pub fn installed_build_depends(
     status_path: &Path,
     build_depends_fields: &[&str],
 ) -> Result<String, Box<dyn std::error::Error>> {
-    let db = StatusDb::load(status_path)?;
+    let content = std::fs::read_to_string(status_path)
+        .map_err(|e| format!("cannot read status file '{}': {}", status_path.display(), e))?;
+    installed_build_depends_from_content(&content, build_depends_fields)
+        .map_err(|e| e.into())
+}
+
+/// Compute the `Installed-Build-Depends` value from the textual content of a
+/// dpkg status database (used when the database lives in another context,
+/// e.g. inside a chroot).
+pub fn installed_build_depends_from_content(
+    status_content: &str,
+    build_depends_fields: &[&str],
+) -> Result<String, String> {
+    let db = StatusDb::from_str(status_content);
 
     let mut work: VecDeque<String> = VecDeque::new();
     for name in &db.essential {