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/debian/mod.rs

@@ -19,7 +19,10 @@ pub mod deps;
 pub mod files;
 pub mod version;
 
-pub use changelog::{ChangelogEntry, parse_changelog_entry};
+pub use changelog::{
+    ChangelogEntry, parse_changelog_entry, parse_changelog_entry_from_str,
+    parse_previous_version_from_str,
+};
 pub use checksums::{Entry as ChecksumEntry, FileChecksums};
 pub use control::{ControlInfo, Paragraph, parse_paragraphs, write_paragraph};
 pub use files::{FilesEntry, FilesList};