Stop decompressing entire archives to get one file — introducing ARCX

ARCX is a new compressed archive format that combines cross-file compression with indexed random access, allowing retrieval of a single file from a large archive in ~7ms without decompressing the entire archive. It achieves up to 200x less data read compared to tar+zstd while maintaining compression ratios within ~3%, making it suitable for CI/CD pipelines, cloud storage, and package registries.