feat: add metapackage build support and fail-fast custom scripts

- add BuildType::Meta handling in builder and interactive spec output
- allow metapackage specs without sources and expose is_metapackage()
- avoid build deps in install planning for metapackages
- run custom non-function build.sh scripts with `sh -e` to stop on first failure
- add regression test for custom script fail-fast behavior
- apply small cleanup refactors (let-chains/iterator simplifications)

src/package/interactive.rs

@@ -21,6 +21,7 @@ impl fmt::Display for BuildType {
             BuildType::Rust => write!(f, "Rust"),
             BuildType::Makefile => write!(f, "Makefile"),
             BuildType::Bin => write!(f, "Binary installer"),
+            BuildType::Meta => write!(f, "Metapackage"),
         }
     }
 }
@@ -624,6 +625,7 @@ pub fn spec_to_minimal_toml(spec: &PackageSpec) -> anyhow::Result<String> {
         BuildType::Rust => "rust",
         BuildType::Makefile => "makefile",
         BuildType::Bin => "bin",
+        BuildType::Meta => "meta",
     };
     build_tbl.insert("type".into(), Value::String(build_type.to_string()));