build,debian: extract reusable Debian format primitives from build/

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.

src/build/changes.rs

@@ -4,9 +4,9 @@
 
 use std::path::Path;
 
-use super::checksums::FileChecksums;
-use super::control::{write_paragraph, Paragraph};
-use super::files::FilesList;
+use crate::debian::checksums::FileChecksums;
+use crate::debian::control::{write_paragraph, Paragraph};
+use crate::debian::files::FilesList;
 
 /// Everything needed to render a `.changes` file.
 #[derive(Debug, Clone)]
@@ -178,7 +178,7 @@ mod tests {
         checksums.add_file(&dsc_path).unwrap();
 
         let mut files_list = FilesList::new();
-        files_list.add(super::super::files::FilesEntry::new(
+        files_list.add(crate::debian::FilesEntry::new(
             "pkg_1.0.dsc",
             "utils",
             "optional",