debian/version: dpkg-compatible version comparison

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 >>.

src/build/mod.rs

@@ -909,6 +909,46 @@ mod differential_tests {
         diff_arch_env_one(None);
     }
 
+    /// Differential check of [`crate::debian::version`] against real
+    /// `dpkg --compare-versions` over every ported dpkg test vector and
+    /// every relation operator.
+    #[test]
+    fn diff_version_compare_against_dpkg() {
+        let vectors = crate::debian::version::test_vectors::COMPARE;
+        assert!(!vectors.is_empty());
+        for (a, b, expected) in vectors {
+            let va = crate::debian::DebianVersion::parse(a)
+                .unwrap_or_else(|e| panic!("parse {a}: {e}"));
+            let vb = crate::debian::DebianVersion::parse(b)
+                .unwrap_or_else(|e| panic!("parse {b}: {e}"));
+            let ours = match va.cmp(&vb) {
+                std::cmp::Ordering::Less => -1,
+                std::cmp::Ordering::Equal => 0,
+                std::cmp::Ordering::Greater => 1,
+            };
+            assert_eq!(ours, *expected, "native compare: {a} cmp {b}");
+
+            // Cross-check the relation operators against the real tool.
+            for (op, holds) in [
+                ("<<", *expected < 0),
+                ("<=", *expected <= 0),
+                ("=", *expected == 0),
+                (">=", *expected >= 0),
+                (">>", *expected > 0),
+            ] {
+                let output = Command::new("dpkg")
+                    .args(["--compare-versions", "--", a, op, b])
+                    .status()
+                    .expect("run dpkg --compare-versions");
+                assert_eq!(
+                    output.success(),
+                    holds,
+                    "dpkg --compare-versions -- {a} {op} {b}"
+                );
+            }
+        }
+    }
+
     #[test]
     fn diff_native_minimal() {
         differential_case(&FixtureSpec::new("pkh-diff-a", "1.0-1", "unstable"));