deb: pin the cross variant choice of M-A:same build-deps in tests

An unqualified Multi-Arch: same build-dependency resolves to the
host-architecture variant only, even when a build-architecture
candidate exists: that is what dpkg's checker accepts (KnownFacts
accepts foreign, host and all instances) and what sbuild installs,
whose dummy package carries the parsed Build-Depends unqualified and
whose sbuild-cross-resolver only filters foreign M-A:foreign and
Essential packages out of the apt universe — verified by resolving a
sbuild-style dummy:arm64 against the resolute/stonking indexes, which
yields libelf-dev:arm64 & co and never the amd64 variants.

The build-architecture side of such a library is only ever installed
through an explicit :native declaration.  Pin both rules so a resolver
regression towards dual-installing unqualified dependencies cannot
reappear unnoticed.

src/deb/local.rs

@@ -1154,6 +1154,39 @@ mod tests {
         assert_eq!(spec, "t:amd64");
     }
 
+    /// Cross parity with dpkg's checker and sbuild's apt-resolved dummy
+    /// package: an unqualified Multi-Arch: same name resolves to the
+    /// host-architecture variant even when a build-architecture candidate
+    /// exists — the build-architecture side is only ever installed
+    /// through an explicit `:native` declaration (or a pkh quirk
+    /// injecting one), never alongside the host variant.
+    #[test]
+    fn install_spec_unqualified_same_package_never_dual_installs() {
+        let host = "arm64";
+        let build = "amd64";
+
+        let spec = install_spec_for(
+            &rel("t"),
+            &[cand("amd64", "same"), cand("arm64", "same")],
+            build,
+            host,
+        )
+        .unwrap();
+        assert_eq!(spec, "t:arm64");
+
+        // The build-architecture side comes from `:native` declarations:
+        // the co-installable build-arch variant of the same source
+        // package.
+        let spec = install_spec_for(
+            &rel("t:native"),
+            &[cand("amd64", "same"), cand("arm64", "same")],
+            build,
+            host,
+        )
+        .unwrap();
+        assert_eq!(spec, "t:amd64");
+    }
+
     /// Qualified dependencies install exactly the variant their qualifier
     /// selects per dpkg semantics; virtual names (no candidates) pass
     /// through unqualified for apt to resolve a provider.