Restore PHP 8.1 randomizer compatibility

[Gruven]

Summary

• add the Arokettu Random extension polyfill for PHP 8.1
• keep native Randomizer::getFloat() on PHP 8.3+ and use an exact gamma-section fallback on PHP 8.1/8.2
• verify the fallback against deterministic sequences captured from PHP 8.3

Testing

• full Docker PHPUnit suite on PHP 8.1: 3680 tests, 16511 assertions, 2 skipped
• targeted analyzer and parity tests on PHP 8.1, 8.2, and 8.3
• Docker PHPStan analysis
• Docker PHP_CodeSniffer standards
• Composer validation