feat: add phase 1 visible stamp layouts

[vitormattos]

Summary

• add phase 1 visible stamp scenario coverage with 4 PDF-generating preview tests
• extend the layout engine with structured support for percentages, absolute positioning, and flex containers while preserving the legacy linear path
• document the new layout subset and share PNG fixture generation across tests

Validation

• docker compose run --rm php vendor-bin/phpunit/vendor/phpunit/phpunit/phpunit
• docker compose run --rm php composer lint
• docker compose run --rm php composer run performance:check