Ruby 4.0

Ruby 4.0 is a milestone release marking the 30th anniversary of the language, focusing on runtime evolution and concurrency. A major highlight is the introduction of Ruby::Box, which provides isolated, container-like execution environments in a single process. This version also features the experimental method-based ZJIT compiler and an optimized, production-ready Ractor concurrency API. Additionally, the widely used Set and Pathname libraries are now promoted to core classes for enhanced, out-of-the-box performance.