feat(package): implement as_package_interface upcast

Make the Rust-only as_package_interface a required trait method and
return self from each concrete implementation, replacing the todo!()
default that PHP needs no equivalent for (implicit subtype coercion).

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

1 files changed, 8 insertions, 0 deletions

diff --git a/crates/shirabe/src/package/root_package.rs b/crates/shirabe/src/package/root_package.rs
index fc04d25..9cdc6e2 100644
--- a/crates/shirabe/src/package/root_package.rs
+++ b/crates/shirabe/src/package/root_package.rs
@@ -129,6 +129,10 @@ impl RootPackageInterface for RootPackage {
     fn set_extra(&mut self, extra: IndexMap<String, PhpMixed>) {
         self.inner.inner.set_extra(extra);
     }
+
+    fn as_package_interface(&self) -> &dyn PackageInterface {
+        self
+    }
 }
 
 impl CompletePackageInterface for RootPackage {
@@ -231,6 +235,10 @@ impl CompletePackageInterface for RootPackage {
     fn set_archive_excludes(&mut self, excludes: Vec<String>) {
         CompletePackageInterface::set_archive_excludes(&mut self.inner, excludes)
     }
+
+    fn as_package_interface(&self) -> &dyn PackageInterface {
+        self
+    }
 }
 
 impl std::fmt::Display for RootPackage {