v4.0.0

• Add deterministic weighted bucket assignment through getBucketForSubject() so users, accounts, and sessions remain in stable experiment cohorts.
• Allow an injectable random-integer generator for reproducible tests while using random_int() on current PHP and a compatible mt_rand() fallback on older PHP.
• Reject empty, non-string, and header-unsafe bucket URLs and allow explicit integer 300–399 redirect status codes.
• Correct the advertised PHP minimum from impossible PHP 5.3 support to the source’s real PHP 5.4 syntax requirement.
• Replace obsolete Travis/Coveralls configuration with passing PHP 5.4 and PHP 8.5 GitHub Actions jobs.
• Document stable assignments, deterministic testing, redirect safety, compatibility, and the v4 upgrade behavior.