Expressions container

[brynpickering]

Partially closes #881

Note

Initial implementation in #871 was human-generated. I've cherry-picked that and then iterated on it with Claude to close some implementation gaps and to add tests/docs.

Note

This does not address the concept of lazy expressions from #881 as I wanted to keep this PR lightweight and less controversial.

Changes proposed in this Pull Request

Model.add_expressions registers a LinearExpression or QuadraticExpression under a name (auto-generated as expr0, expr1, ... if omitted), accessible afterwards via Model.expressions (an Expressions container mirroring Model.variables/Model.constraints) and removable via Model.remove_expressions.
Named expressions are persisted by Model.to_netcdf/linopy.read_netcdf and preserved by Model.copy, copy.copy, copy.deepcopy, and pickling.

Checklist

• AI-generated content is marked (see AGENTS.md).
• Code changes are sufficiently documented; i.e. new functions contain docstrings and further explanations may be given in doc.
• Unit tests for new features were added (if applicable).
• A note for the release notes doc/release_notes.rst of the upcoming release is included.
• I consent to the release of this PR's code under the MIT license.