build: re-implement source builds natively, drop dpkg-buildpackage shell-out

Replace the 'dpkg-buildpackage -S' wrapper with a native pipeline in
src/build/:

- deb822 control parser/writer with dpkg-compatible multiline rendering
  (control.rs)
- md5/sha1/sha256 checksum registry, insertion-ordered like dpkg's
  artifact accumulation (checksums.rs)
- Debian version splitting/validation and full changelog entry parsing,
  including binNMU binary-only entries (metadata.rs)
- build-type bitflags and rules-target/artifact-suffix mapping
  (buildtype.rs)
- environment setup: SOURCE_DATE_EPOCH, DEB_BUILD_OPTIONS,
  dpkg-architecture env dump, vendor default profiles and the sanitized
  Environment field recorded in .buildinfo (env.rs)
- debian/files registry with atomic saves (files.rs)
- native .buildinfo writer, including the Installed-Build-Depends
  closure computed over the dpkg status database (buildinfo.rs)
- native .changes writer emitting dpkg's canonical field order with
  legacy Files + Checksums-Sha1/Sha256 (changes.rs)
- gpgme clearsigning with the transitive checksum cascade
  (dsc -> buildinfo -> changes), key discovery from the changelog
  maintainer and UNRELEASED no-sign handling (sign.rs)

dpkg-source (-b/--before-build/--after-build) intentionally remains a
subprocess; debian/rules execution is unchanged.

Validated differentially against real dpkg-buildpackage -S -I -i -nc -d
on native and 3.0 (quilt) fixture packages: .dsc byte-identical, .changes
payload matches modulo machine-dependent Installed-Build-Depends and
Environment content, all signatures verify with gpg, artifact ordering
and UNRELEASED no-sign behavior match dpkg.

src/build.rs

@@ -1,82 +0,0 @@
-use std::error::Error;
-use std::path::Path;
-use std::process::Command;
-
-use crate::changelog::parse_changelog_footer;
-use crate::utils::gpg;
-
-/// Build a Debian source package (to a .dsc)
-pub fn build_source_package(cwd: Option<&Path>) -> Result<(), Box<dyn Error>> {
-    let cwd = cwd.unwrap_or_else(|| Path::new("."));
-
-    // Parse changelog to get maintainer information from the last modification entry
-    let changelog_path = cwd.join("debian/changelog");
-    let (maintainer_name, maintainer_email) = parse_changelog_footer(&changelog_path)?;
-
-    // Check if a GPG key matching the maintainer's email exists
-    let signing_key = match gpg::find_signing_key_for_email(&maintainer_email) {
-        Ok(key) => key,
-        Err(e) => {
-            // If GPG is not available or there's an error, continue without signing
-            log::warn!("Failed to check for GPG key: {}", e);
-            None
-        }
-    };
-
-    // Build command arguments
-    let mut command = Command::new("dpkg-buildpackage");
-    command
-        .current_dir(cwd)
-        .arg("-S")
-        .arg("-I")
-        .arg("-i")
-        .arg("-nc")
-        .arg("-d");
-
-    // If a signing key is found, use it for signing
-    if let Some(key_id) = &signing_key {
-        command.arg(format!("--sign-keyid={}", key_id));
-        log::info!("Using GPG key {} for signing", key_id);
-    } else {
-        command.arg("--no-sign");
-        log::info!(
-            "No GPG key found for {} ({}), building without signing",
-            maintainer_name,
-            maintainer_email
-        );
-    }
-
-    let status = command.status().map_err(|e| {
-        format!(
-            "Failed to run 'dpkg-buildpackage': {}. \
-             Is 'dpkg-dev' (which provides dpkg-buildpackage) installed?",
-            e
-        )
-    })?;
-
-    if !status.success() {
-        return Err(format!(
-            "dpkg-buildpackage failed with status: {}. \
-             Re-run with 'RUST_LOG=debug' for more details, or run \
-             'dpkg-buildpackage -S -I -i -nc -d' manually in '{}' to see the full output.",
-            status,
-            cwd.display()
-        )
-        .into());
-    }
-
-    if signing_key.is_some() {
-        println!("Package built and signed successfully!");
-    } else {
-        println!("Package built successfully (unsigned).");
-    }
-
-    Ok(())
-}
-
-#[cfg(test)]
-mod tests {
-
-    // We are not testing the build part, as for now this is just a wrapper
-    // around dpkg-buildpackage.
-}