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/builder/mod.rs

@@ -226,6 +226,12 @@ pub fn build(
         BuildType::Python => python::build(spec, src_dir, destdir, cross, export_compiler_flags),
         BuildType::Rust => rust::build(spec, src_dir, destdir, cross, export_compiler_flags),
         BuildType::Bin => bin::build(spec, src_dir, destdir, cross, export_compiler_flags),
+        BuildType::Meta => {
+            // Metapackages are metadata-only; create an empty staging root and let
+            // packaging/installation metadata carry dependencies.
+            std::fs::create_dir_all(destdir)?;
+            Ok(())
+        }
         BuildType::Makefile => {
             makefile::build(spec, src_dir, destdir, cross, export_compiler_flags)
         }