checksums: add SHA-512 and a Checksums-* field parser

The checksum model only carried md5/sha1/sha256 while deb-buildinfo(5)
defines Checksums-Sha512, and there was no way to parse a Checksums-*
field body back into entries. Add ChecksumKind::Sha512 (computed
alongside the others), a field parser validating the
'<hex> <size> <name>' grammar, and an only-if-populated
Checksums-Sha512 emission in .buildinfo — deliberately dormant in the
dpkg-parity flows, which never emit it, and .dsc/.changes untouched.

src/build/binary.rs

@@ -110,6 +110,9 @@ pub fn generate_binary_metadata(
         let entry_hashes = hashes
             .remove(name)
             .ok_or_else(|| format!("artifact '{name}' listed in debian/files but not found"))?;
+        // SHA-512 stays unknown here: like dpkg-genbuildinfo, no SHA-512
+        // digest is computed for the artifacts, and an empty digest keeps
+        // the `Checksums-Sha512` field of the `.buildinfo` omitted.
         checksums.insert_entry(
             name,
             ChecksumEntry {
@@ -117,6 +120,7 @@ pub fn generate_binary_metadata(
                 md5: entry_hashes.md5,
                 sha1: entry_hashes.sha1,
                 sha256: entry_hashes.sha256,
+                sha512: String::new(),
             },
         );
         // Architecture accumulation in encounter order (dpkg-genchanges).
@@ -285,6 +289,9 @@ pub fn generate_binary_metadata(
                 md5: h.md5.clone(),
                 sha1: h.sha1.clone(),
                 sha256: h.sha256.clone(),
+                // No SHA-512 digest available (see above); keeps the
+                // `Checksums-Sha512` `.buildinfo` field omitted.
+                sha512: String::new(),
             },
         );
     }
@@ -457,6 +464,8 @@ fn include_dsc_artifacts(
                 md5: h.md5.clone(),
                 sha1: h.sha1.clone(),
                 sha256: h.sha256.clone(),
+                // No SHA-512 digest available (see above).
+                sha512: String::new(),
             },
         );
     }
@@ -474,6 +483,9 @@ fn include_dsc_artifacts(
                 md5: p.md5.clone().unwrap_or_default(),
                 sha1: p.sha1.clone().unwrap_or_default(),
                 sha256: p.sha256.clone().unwrap_or_default(),
+                // The `.dsc` records no SHA-512 (dpkg only writes
+                // sha1/sha256 there).
+                sha512: String::new(),
             },
         );
     }