chore(deps): combined dependency upgrades

[powli]

Summary

This PR combines multiple valid dependency upgrades into a single update:

PHP Dependencies

• PHPStan: ^1.10 → ^2.0 (PR chore(deps): update dependency phpstan/phpstan to v2 - autoclosed #9 - all CI checks passing)

GitHub Actions

• actions/checkout: v4.2.2 → v6.0.1 (PR chore(deps): update actions/checkout action to v6 - autoclosed #7)
• actions/cache: v4 → v5 (PR chore(deps): update actions/cache action to v5 - autoclosed #6)
• codecov/codecov-action: v4 → v5 (PR chore(deps): update codecov/codecov-action action to v5 - autoclosed #8)
• shivammathur/setup-php: digest update (PR chore(deps): update shivammathur/setup-php digest to 44454db #3)

Excluded PRs

PR	Reason
#4 (checkout v4.3.1)	Superseded by v6 upgrade in PR #7
#11 (PHPUnit v12)	Breaks PHP 8.2 support (requires PHP 8.3+)
#12 (Symfony v8)	CI failing - needs constraint syntax fix

Test plan

• CI pipeline passes all checks (PHP 8.2, 8.3, 8.4 × Symfony 5.4, 6.4, 7.1)
• PHPStan analysis passes with v2
• Code coverage job completes successfully

Closes #3, #6, #7, #8, #9