Declare Singleton::SingletonInstanceMethods

[infiton]

Summary

Declare the internal Singleton::SingletonInstanceMethods module in the singleton stdlib signatures.

Ruby 3.4 and later expose this module as part of Singleton's implementation. Reflection-based signature generators can therefore produce declarations such as:

class Registry
  include Singleton::SingletonInstanceMethods
end

RBS currently treats this as an unknown mixin because only the related Singleton::SingletonClassMethods name is declared.

The new declaration is intentionally empty. Its methods are already represented directly on Singleton; registering the name prevents mechanically generated signatures from failing validation. This matches the existing treatment of SingletonClassMethods, added in #978.

Tests

2 tests, 9 assertions, 0 failures, 0 errors

The singleton library signatures also pass rbs validate.