debian/arch: native architecture tables replacing dpkg-architecture

Embed dpkg's factual cputable/ostable/tupletable/abitable data and
implement tuple/triplet/multiarch lookups, wildcard matching, arch
restriction evaluation and the full DEB_BUILD_*/DEB_HOST_*/DEB_TARGET_*
environment dump natively.

build/env.rs::arch_env now delegates to the native implementation
instead of shelling out to 'dpkg-architecture -f'.

Differential gate (build/mod.rs): arch_env(Some(a)) must equal real
'dpkg-architecture -f -a a' key-for-key for every architecture listed by
'dpkg-architecture -L', plus the native case. Data tables carry upstream
attribution comments; no dpkg code was transliterated.

src/build/mod.rs

@@ -859,6 +859,56 @@ mod differential_tests {
         differential_on_tree(&tree);
     }
 
+    /// Differential check of [`crate::debian::arch::arch_env`] against real
+    /// `dpkg-architecture -f -a <arch>` for one architecture.
+    fn diff_arch_env_one(arch: Option<&str>) {
+        let mut cmd = Command::new("dpkg-architecture");
+        cmd.arg("-f");
+        if let Some(a) = arch {
+            cmd.args(["-a", a]);
+        }
+        let output = cmd.output().expect("run dpkg-architecture (is dpkg-dev installed?)");
+        assert!(
+            output.status.success(),
+            "dpkg-architecture -f {arch:?} failed: {}",
+            String::from_utf8_lossy(&output.stderr)
+        );
+        let mut expected = BTreeMap::new();
+        for line in String::from_utf8_lossy(&output.stdout).lines() {
+            if let Some((key, value)) = line.split_once('=') {
+                expected.insert(key.to_string(), value.to_string());
+            }
+        }
+
+        let ours = crate::debian::arch::arch_env(arch)
+            .unwrap_or_else(|e| panic!("native arch_env({arch:?}) failed: {e}"));
+        assert_eq!(ours, expected, "arch_env({arch:?}) differs from dpkg-architecture");
+    }
+
+    /// Every architecture known to the local dpkg must produce an identical
+    /// environment dump (`dpkg-architecture -L`).
+    #[test]
+    fn diff_arch_env_all_known_arches() {
+        let output = Command::new("dpkg-architecture")
+            .arg("-L")
+            .output()
+            .expect("run dpkg-architecture -L (is dpkg-dev installed?)");
+        assert!(output.status.success());
+        for arch in String::from_utf8_lossy(&output.stdout).lines() {
+            let arch = arch.trim();
+            if arch.is_empty() {
+                continue;
+            }
+            diff_arch_env_one(Some(arch));
+        }
+    }
+
+    /// Native (no explicit host architecture) must match too.
+    #[test]
+    fn diff_arch_env_native() {
+        diff_arch_env_one(None);
+    }
+
     #[test]
     fn diff_native_minimal() {
         differential_case(&FixtureSpec::new("pkh-diff-a", "1.0-1", "unstable"));