feat(installer): register default installers after Composer Rc is built

LibraryInstaller and PluginInstaller upgrade the Composer back-reference
in their constructors, so they could not be built inside Rc::new_cyclic
where the weak handle is not yet upgradeable. Defer create_default_
installers until after the cyclic Rc is established, where the weak
handle resolves, and implement it to register Library -> Plugin ->
Metapackage with a single shared BinaryInstaller.

To share one BinaryInstaller (as Composer does), LibraryInstaller's
binary_installer becomes Rc<RefCell<dyn BinaryInstallerInterface>>
instead of an owned Box; PluginInstaller and the __set_binary_installer
test seam follow.

This clears "Unknown installer type: metapackage". Un-ignores the six
remove tests that now pass; the remaining install/remove tests are
re-labeled for the next blocker (InstallationManager::execute_batch
still leaves the install/cleanup/repo.write promise chain as a todo!()
stub, so package operations do not actually execute).

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

1 files changed, 5 insertions, 3 deletions

diff --git a/crates/shirabe/tests/installer/library_installer_test.rs b/crates/shirabe/tests/installer/library_installer_test.rs
index 42730fe..f519764 100644
--- a/crates/shirabe/tests/installer/library_installer_test.rs
+++ b/crates/shirabe/tests/installer/library_installer_test.rs
@@ -366,9 +366,11 @@ fn test_ensure_binaries_installed() {
         None,
         None,
     );
-    library.__set_binary_installer(Box::new(RecordingBinaryInstaller {
-        calls: calls.clone(),
-    }));
+    library.__set_binary_installer(std::rc::Rc::new(std::cell::RefCell::new(
+        RecordingBinaryInstaller {
+            calls: calls.clone(),
+        },
+    )));
     let package = get_package("foo/bar", "1.0.0");
     let expected_path = library.get_install_path(package.clone()).unwrap();