Foundations: PHP 8.4 / Laravel 12+, CI matrix, coverage and codestyle #1

Bring the repo up to the next-gen Laravel standards (see orchestration/STANDARDS.md).

• Start from Javaabu/package-skeleton: javaabu/block-editor, namespace Javaabu\\…
• Versions: "php": "^8.4", illuminate/*: ^12.0 || ^13.0, Testbench ^10.0 || ^11.0
• Javaabu codestyle (boost-javaabu-guidelines) enforced with Pint in CI
• CI matrix: PHP 8.4 and 8.5 × Laravel 12 and 13 × prefer-lowest and prefer-stable
• Coverage uploaded to Codecov from one leg
• README badges: tests, Codecov, Packagist, licence