Handle extend inside class << self for rank-N singletons

[soutaro]

• Linearize: drop the !is_singleton_class guard so an extend M written inside a class << self always creates that singleton's own singleton (the rank-2 Foo::<Foo>::<<Foo>>), with M mixed in as an ancestor. It was previously skipped for singleton classes.
• Seed rank-N materialization from any singleton whose own class << self carries an extend: the extend mixes the module into that singleton's own singleton as an ancestor (mixins are never members), so the definition/member checks that detect explicit singletons miss it. Seeding here materializes the rank-(N + 1) singleton for every subclass too.
• Add tests: rank-2 singleton for the extended class itself, and for a subclass.