Return all source fields by default and test PHP 8.5

[Jord-JD]

Summary

• make the documented empty field selection return all fields for every source type
• allow PSR-6 cache implementations using psr/cache 1, 2, or 3
• add PHP 7.1 through 8.5 coverage and current PHPUnit support
• document direct source reads

Validation

• existing unit and integration suite plus a direct CSV regression test across the compatibility matrix