`attr_reader`, `attr_writer`, and `attr_accessor` written inside a
singleton class (e.g. `class << self`) were always treated as instance
methods, so

    class C
      class << self
        attr_accessor :age
      end
    end

was dumped as `def age` / `def age=` instead of `def self.age` /
`def self.age=`.

The three attr meta nodes ignored the enclosing scope and hardcoded
`false` for the singleton flag when resolving the backing ivar and
defining the methods. Detect the metaclass scope the same way `DefNode`
does (`lenv.cref.scope_level == :metaclass`) and thread it through, so
the accessors and their ivar are defined on the singleton. This also
covers `class << SomeConstant`.

Promote scenario/known-issues/eigenclass-attr.rb to scenario/class/.

Co-authored-by: Claude Opus 4.8 (1M context) <[email protected]>