ci: test the full supported server matrix

[twme-ai]

Summary

• start every stable Paper and Folia release from 1.19.4 through 26.2 in CI
• run Mineflayer functional checks at the 1.19, 1.20, and 1.21 protocol generations
• group compatible runtime cases by Java version to keep the matrix maintainable
• document Paper-first runtime coverage and local Spigot BuildTools checkpoints

Local verification

• JDK 25 Maven build and 8 unit tests
• all stable Paper and Folia versions in the supported range started with DebugStickPro enabled
• Mineflayer command, MiniMessage item, and VirtualEntities spawn/removal checks at protocol checkpoints
• official BuildTools Spigot 1.20.6 functional test and Spigot 1.21.11 startup test
• YAML, shell, Node syntax, and diff checks