Add health insurance premiums to local area calibration

[baogorek]

Summary

Add support for health_insurance_premiums_without_medicare_part_b as a calibration target in the local area geo-stacking workflow.

Details

• This is a person-level variable that needs to be aggregated to household level for calibration
• The existing sparse_matrix_builder.py already supports this through PolicyEngine's map_to="household" parameter
• Added runner script build_calibration_matrix.py for building calibration matrices with SNAP + health insurance targets
• Added test coverage in test_sparse_matrix_builder.py

Changes

• build_calibration_matrix.py: Runner script for building X_sparse matrix and target vector
• test_sparse_matrix_builder.py: Tests for person-level aggregation and combined targets
• changelog_entry.yaml: Updated with new feature