Move the generic components out of src/build/ into a new src/debian/
module so they can be reused independently of the build pipeline:
deb822 control parsing (plus the debian/control model), file checksum
registry, debian/files registry, Debian version handling and changelog
entry parsing.
Merge OpenPGP clearsigning into utils/gpg.rs next to the existing key
discovery helper, making signing available outside of builds.
Delegate changelog.rs header/footer parsing to the new
debian::changelog parser, removing the duplicate regex implementation.
src/build/ keeps only build-specific logic: the pipeline driver,
build types, environment setup and the .buildinfo/.changes writers.
