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/custom.rs

@@ -113,7 +113,9 @@ pub fn build(
             build_function_mode_command(spec, destdir, &abs_build_script)?
         } else {
             let mut cmd = fakeroot::wrap_install_command("sh", destdir);
-            cmd.arg(&abs_build_script);
+            // Run custom scripts with `-e` so command failures stop the build immediately
+            // instead of being masked by later shell commands.
+            cmd.arg("-e").arg(&abs_build_script);
             cmd
         };
         cmd.current_dir(&build_dir);
@@ -422,4 +424,32 @@ depot_install() {
         assert!(err.to_string().contains("Custom build script failed"));
         Ok(())
     }
+
+    #[test]
+    fn test_build_non_function_mode_stops_on_first_command_failure() -> Result<()> {
+        let tmp_src = tempdir()?;
+        let tmp_dest = tempdir()?;
+
+        let build_sh = tmp_src.path().join("build.sh");
+        std::fs::write(
+            &build_sh,
+            r#"#!/bin/sh
+false
+exit 0
+"#,
+        )?;
+        #[cfg(unix)]
+        {
+            use std::os::unix::fs::PermissionsExt;
+            let mut perms = std::fs::metadata(&build_sh)?.permissions();
+            perms.set_mode(0o755);
+            std::fs::set_permissions(&build_sh, perms)?;
+        }
+
+        let spec = mk_spec("custom-fail-fast", "1.0");
+        let err = build(&spec, tmp_src.path(), tmp_dest.path(), None, true)
+            .expect_err("non-function custom scripts should fail when a command fails");
+        assert!(err.to_string().contains("Custom build script failed"));
+        Ok(())
+    }
 }

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)
         }