Improve documentation, add PHP 8.1-8.5 CI matrix, and refactor codebase

[lazerg]

This PR improves developer experience with comprehensive inline documentation and ensures compatibility across all supported PHP versions.

Documentation

• Add docblocks with @param, @return, and @example to all trait methods
• Each example uses DifficultyEnum showing real return values for quick IDE reference
• Rewrite README with cleaner structure: installation, enum example, and organized method sections

CI & Compatibility

• Update GitHub Actions to test against PHP 8.1, 8.2, 8.3, 8.4, 8.5
• Fix PHP 8.1 compatibility by adding Pest v1.x support (Pest v2+ requires PHP 8.2)
• Use shivammathur/setup-php action for reliable PHP version matrix

Code Quality

• Add TooManyRandomValuesException for random() when count exceeds available cases
• Add UndefinedCaseException for __callStatic() when case name doesn't exist
• Refactor tests to use Pest it() syntax with descriptive names
• Add DifficultyEnum test fixture used across all test files