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>

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