Implement the documented dpkg ordering algorithm (Debian Policy 5.6.1):
numeric epoch, then upstream/revision compared as alternating non-digit
and digit chunks, with '~' ordering before anything including the empty
chunk and letters before non-letters in non-digit chunks.
Adds Ord/PartialOrd for DebianVersion, a free compare() and a
later_than() convenience.
Unit tests port all vectors from dpkg's scripts/t/Dpkg_Version.t plus
Ubuntu-flavored cases (security uploads, ~ppa1 backports). Differential
gate: every vector cross-checked against real 'dpkg --compare-versions'
for <<, <=, =, >= and >>.
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.
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