Adopt EUPL-1.2 licence for the project

Add LICENSE (full EUPL-1.2 text + project copyright), CONTRIBUTING.md
with the inbound = outbound rule, and declare the licence in
pyproject.toml and README.md.

Co-Authored-By: Claude Opus 4.7 <noreply@anthropic.com>

CONTRIBUTING.md

@@ -0,0 +1,66 @@
+# Contributing to testium
+
+Thank you for your interest in contributing to testium.
+
+## License of contributions
+
+testium is licensed under the **European Union Public Licence v. 1.2 (EUPL-1.2)** —
+see the [LICENSE](LICENSE) file at the repository root.
+
+By submitting a contribution to this project (pull request, patch, issue
+attachment, or any other form of code, documentation or media), you agree
+that your contribution is licensed to the project and to the public under the
+**same EUPL-1.2** terms (or any later version of the EUPL approved by the
+European Commission), and you certify that:
+
+- you are the author of the contribution, or you have the right to submit it
+  under the EUPL-1.2;
+- to the best of your knowledge, the contribution does not infringe any
+  third-party intellectual-property rights;
+- the contribution may be redistributed by the project under the EUPL-1.2 and
+  any compatible licence listed in the EUPL-1.2 Appendix.
+
+This is the **inbound = outbound** rule: contributions come in under the same
+licence the project ships under.
+
+You retain copyright on your contribution. The project does **not** ask you
+to sign a CLA or assign your copyright.
+
+## SPDX header in new source files
+
+When creating a new source file, please include the following header at the
+top of the file (adjust the comment marker to the file's language):
+
+```python
+# SPDX-License-Identifier: EUPL-1.2
+# Copyright (c) <year> <your name>
+```
+
+For existing files, keep the header that is already there.
+
+## How to contribute
+
+1. Open an issue describing the change you want to make (bug, feature, doc).
+2. Fork the repository, create a topic branch.
+3. Commit with a clear message (one logical change per commit).
+4. Make sure the validation suite still passes:
+   ```
+   ./run.sh -b -l mon_log.log -- test/validation/main.tum
+   ```
+5. Open a pull request against `main`.
+
+## Coding conventions
+
+- Python ≥ 3.11
+- Follow existing style in the file you are modifying
+- Add or update tests in `test/validation/` for new test items or behaviours
+- Update `CLAUDE.md` and the Sphinx manual for user-visible changes
+
+## Reporting security issues
+
+Please do **not** report security vulnerabilities through public GitHub
+issues. Instead, send an email to the project maintainer directly.
+
+## Questions
+
+Open a GitHub Discussion or an issue tagged `question`.