CI: Validate release prerequisites before packaging

[darknoon29]

Problem

The release workflow installs dependencies with composer install --no-dev. PHPUnit is a development dependency, so the release pipeline cannot validate the test suite before creating a production package.

Affected code

• .github/workflows/release.yml
• composer.json

Expected outcome

A release fails before packaging when dependency validation or automated tests fail, while the final package still excludes development dependencies.

Acceptance criteria

• Add a release-validation stage that installs full dependencies.
• Run composer validate --strict and the PHPUnit suite in that stage.
• Only create the distributable package after validation succeeds.
• Retain a separate production dependency installation with --no-dev for the package contents.
• Ensure release artifacts do not include vendor/ development-only dependencies.

Scope

This complements, but does not replace, the pull-request test workflow.