Configures a CI matrix that runs tests across operating systems (Linux / macOS / Windows) and runtime versions (Node 18/20/22; Python 3.10/3.11/3.12; Java 17/21; .NET 6/8). Wires GitHub Actions matrix syntax, addresses OS-specific quirks (path separators, line endings, file permissions). Use when the product ships across OS / runtime combinations and the team needs continuous cross-platform coverage.