Changes proposed in this Pull Request

Review fixes for #630. Addresses some issues found during code review.

Fixed

• Constraint.__repr__ passed CSR column positions instead of variable labels to print_single_expression, producing wrong output. Now maps back via vlabels[csr.indices[...]].
• Constraints.set_blocks crashed on frozen Constraint — accessed ._data which only exists on MutableConstraint. Now handles both types. -> this is not important for now but will be in case we want to revive the pips interface
• extracted _active_to_dataarray helper on Constraint
• Constraints.reset_dual — simplified to c._dual = None.

Identified but Deferred

• Constraint.__repr__ duplicates ConstraintBase.__repr__ rendering logic
• Constraint called "immutable" but sanitize_* mutates — perhaps we should rename it to "frozen" ?
• VariableLabelIndex.__init__ uses Any instead of Variables
• Constraint.coords triggers silent Dataset reconstruction (no PerformanceWarning)
• Constraints.to_matrix duplicates MatrixAccessor._build_cons stacking
• MatrixAccessor.dual complex isinstance branching
• Constraint.to_polars() return type Any -> pl.DataFrame
• Missing PerformanceWarning emission tests
• coords_to_dataset_vars/coords_from_dataset untested
• Missing loc on frozen Constraint

Checklist

• 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.