Add requirements-reference.txt for reproducible Python deps (fixes #610)

[PranjalManhgaye]

Description

Problem
For each distribution release we need reproducible Python dependency versions, but pinning in every tutorial requirements.txt would require a lot of merge back-and-forth. There is also no explicit list of versions known to work for a given release.

Approach
Introduce a separate requirements-reference.txt (lockfile-style) with pinned versions, without changing the existing tutorial requirements.txt or run scripts. The reference file is only a manifest of versions that are known to work for a distribution.

What changed

• Add tools/tests/requirements-reference.txt with pinned versions (pyprecice, numpy, matplotlib, nutils, setuptools)
• Add update_requirements_reference.py to regenerate it from reference_versions.yaml (pyprecice from PYTHON_BINDINGS_REF)
• Add validate_requirements_reference.py to ensure the pyprecice version matches reference_versions.yaml
• Add GitHub Action check-requirements-reference.yml to run this validation on PR/push
• Document the mechanism in tools/tests/README.md and add a reminder to the release PR template

How to update at release

• Preferred: pip freeze from the systemtest Docker image, then pip freeze | python3 update_requirements_reference.py --from-freeze
• Or: python3 update_requirements_reference.py (uses reference_versions.yaml for pyprecice and script defaults for other packages)

Fixes #610

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Checklist:

• I added a summary of any user-facing changes (compared to the last release) in the changelog-entries/<PRnumber>.md.
• I will remember to squash-and-merge, providing a useful summary of the changes of this PR.