Integrate tests, ruff config and CI from chore/professional-standard #3

Context

main has no tests, no ruff configuration and no CI. The only CI attempt (PR #2, ci/github-actions) fails at Lint with 20 errors and skips compile and tests.

The branch chore/professional-standard holds the full package, never integrated: tests for the 8 scripts, pyproject.toml (ruff: line length 100, rules E, F, I, UP, RUF), pytest.ini, requirements-dev.txt, a CI workflow (ruff check . && ruff format --check ., then pytest -v), CI badge and a development section in the README.

Proposal

Deliver the branch to main through a new PR, with the owner's decision (option A):

• Open the PR from chore/professional-standard and let CI run.
• Fix what CI reports (lint, formatting, tests) on that branch, in small commits, without disabling or skipping any check.
• Merge only with every job green and the tests really executed.
• Afterwards, with the owner's approval: close PR ci: add GitHub Actions workflow (lint, compile, tests) #2, which is superseded, and leave the branches for the owner to decide.

Closes #1 (Add CI with GitHub Actions).

Done when

• CI on the PR is green: Lint (check and format) and pytest -v executed and passing
• Tests for the 8 scripts run in CI
• CI on main is green after merge
• README on main keeps the English translation already merged