v6.0.1

• Add Ruby 4.0 testing. (64e1a0b9)
• Stop allocating an argument array and hash on every size, empty? and ids_reader call of a collection association. Thanks czj (e01f8a83)
• Keep the test suite passing with combustion 1.6+, whose load_defaults now makes belongs_to required. Thanks czj (dcc74ffc)