Modernize PHP tooling for PHP 8

[schlessera]

Summary

• Update Composer PHP/runtime constraints and refresh dev tooling for Composer 2 and PHP 8.3.
• Modernize PHPUnit, PHPStan, PHPCS/WPCS, and Psalm config while preserving the existing project style.
• Fix small PHP 8/static-analysis issues exposed by the refreshed tools.

Links back to Paperclip MWPA-8 and MWPA-11.

Verification

• composer validate --no-check-publish
• composer install
• composer run-script lint
• composer run-script unit
• composer run-script cs
• composer run-script analyze
• vendor/bin/psalm --no-progress

Note: PHPCS exits successfully but reports upstream deprecation warnings for two bundled Squiz sniffs.