Resolve retroactive singleton method visibility changes

[alexcrocha]

Part of #89 and follows #780, which indexed SingletonMethodVisibilityDefinition and left the Definition::SingletonMethodVisibility arm in resolution as a no-op.

This PR resolves retroactive private_class_method and public_class_method calls. We process visibility definitions after the main convergence loop, so the target method declaration is guaranteed to exist by the time we attach visibility.

For inherited targets like private_class_method :foo where foo comes from a parent class's singleton, we create a child-owned MethodDeclaration on the child's singleton class. This matches what #738 did for instance methods, and what CRuby reports when you ask Child.singleton_class.instance_method(:foo).owner:

class Parent
  def self.foo; end
end

class Child < Parent
  private_class_method :foo
end

Child.singleton_class.instance_method(:foo).owner
# => #<Class:Child>   (CRuby copies the method to Child when visibility changes)

Parent.singleton_class.instance_method(:foo).owner
# => #<Class:Parent>  (untouched on the parent)

Why document-scoped diagnostics

When the target doesn't resolve, we emit UndefinedSingletonMethodVisibilityTarget attached to the document, not to the singleton declaration. Walking ancestors via get_or_create_singleton_class(Foo, ...) can create Foo::<Foo> as a side effect, even when it has no real members. For class Foo; private_class_method :missing; end where Foo has no other singleton methods, that synthesized singleton class only exists to host the diagnostic. Attaching to it would leak the declaration on file delete. Document-scoped clears with the file.

In this PR

• add resolve_singleton_method_visibilities, mirroring resolve_method_visibilities
• wire it into handle_remaining_definitions via a new arm
• add the Graph::visibility arm for SingletonMethodVisibility so callers can read it back
• emit UndefinedSingletonMethodVisibilityTarget (document-scoped) when the target is missing
• resolution tests and Ruby integration tests for direct, qualified-receiver, inherited, and undefined-target cases
• regression test verifying the diagnostic clears when the offending file is deleted

[alexcrocha]

Warning

This pull request is not mergeable via GitHub because a downstack PR is open. Once all requirements are satisfied, merge this PR as a stack on Graphite.
Learn more

• Extract shared helpers for visibility-call processing #783
• Support array and inline-def arguments to private_class_method #782
• Resolve retroactive singleton method visibility changes #781 👈 (View in Graphite)
• Index private_class_method / public_class_method calls #780
• main

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