Fix language loading and add resilient lookups - #3

Summary

• fix PSR-4 autoloading by matching the Language class filename
• exclude the CSV header from results and validate/close the bundled data safely
• declare public properties to avoid PHP 8.2 dynamic-property deprecations
• add cached, mutation-safe, trimmed, case-insensitive lookups and underscore-free ISO aliases
• correct the README property examples and document missing-result behavior
• declare the code's real PHP 7.0 minimum without raising it

Verification

• jq empty composer.json
• git diff --check
• GitHub Actions exercises PHP 7.0/Composer 2.2 and PHP 8.5/latest Composer