Add selected loss.py targets to unified calibration

[anth-volk]

Fixes #945

Requesting review from @MaxGhenis to confirm that we want these targets in US.h5 and state/district-level datasets.

Summary

• Port selected legacy loss.py target families into unified calibration target ETL and selection.
• Add SOI taxable AGI-grid targets for irs_employment_income, pension_income, and social_security.
• Add or select state EITC claimant counts, state SNAP household counts, LIHEAP household counts, state ACS rent, real-estate-tax itemizer counts, AOTC and education credit targets, and Medicare Part B age-bucket targets.
• Add focused unit coverage and a Towncrier changelog fragment.

Targets Added

Exact target-selection rules added to policyengine_us_data/calibration/target_config.yaml:

Variable	Geo level	Domain variable
household_count	state	snap
rent	state	none
tax_unit_count	state	eitc
tax_unit_count	state	real_estate_taxes,tax_unit_itemizes
tax_unit_count	national	adjusted_gross_income,income_tax_before_credits,irs_employment_income
irs_employment_income	national	adjusted_gross_income,income_tax_before_credits,irs_employment_income
tax_unit_count	national	adjusted_gross_income,filing_status,income_tax_before_credits,irs_employment_income
irs_employment_income	national	adjusted_gross_income,filing_status,income_tax_before_credits,irs_employment_income
tax_unit_count	national	adjusted_gross_income,income_tax_before_credits,pension_income
pension_income	national	adjusted_gross_income,income_tax_before_credits,pension_income
tax_unit_count	national	adjusted_gross_income,filing_status,income_tax_before_credits,pension_income
pension_income	national	adjusted_gross_income,filing_status,income_tax_before_credits,pension_income
tax_unit_count	national	adjusted_gross_income,income_tax_before_credits,social_security
social_security	national	adjusted_gross_income,income_tax_before_credits,social_security
tax_unit_count	national	adjusted_gross_income,filing_status,income_tax_before_credits,social_security
social_security	national	adjusted_gross_income,filing_status,income_tax_before_credits,social_security
medicare_part_b_premium	national	age
household_count	national	spm_unit_energy_subsidy_reported
refundable_american_opportunity_credit	national	refundable_american_opportunity_credit
education_tax_credits	national	education_tax_credits
tax_unit_count	national	refundable_american_opportunity_credit
tax_unit_count	national	education_tax_credits
tax_unit_count	national	real_estate_taxes,tax_unit_itemizes

New ETL target rows introduced by this PR:

• State EITC claimant counts: tax_unit_count by state where eitc > 0, sourced from IRS EITC Central controls.
• State ACS rent totals: rent by state, sourced from acs_housing_costs_YEAR.csv / ACS B25060.
• Medicare Part B age buckets: medicare_part_b_premium nationally by 10-year age buckets, sourced from healthcare_spending.csv.
• SOI taxable AGI-grid income rows for irs_employment_income, pension_income, and social_security, with matching positive-domain tax_unit_count rows, sourced from tracked SOI Table 1.4 targets.

Verification

• python3 -m py_compile policyengine_us_data/db/etl_irs_soi.py policyengine_us_data/db/etl_national_targets.py policyengine_us_data/db/create_field_valid_values.py tests/unit/test_etl_irs_soi_overlay.py tests/unit/test_etl_national_targets.py tests/unit/calibration/test_target_config.py
• ruff check policyengine_us_data/db/etl_irs_soi.py policyengine_us_data/db/etl_national_targets.py tests/unit/test_etl_irs_soi_overlay.py tests/unit/test_etl_national_targets.py tests/unit/calibration/test_target_config.py
• ruff format --check policyengine_us_data/db/etl_irs_soi.py policyengine_us_data/db/etl_national_targets.py tests/unit/calibration/test_target_config.py tests/unit/test_etl_irs_soi_overlay.py tests/unit/test_etl_national_targets.py
• git diff --check

Notes

• make lint was run but is blocked locally by pre-existing untracked .tmp/ report-card artifacts that ruff sees outside this PR.
• uv run pytest for the targeted tests remains blocked locally on macOS x86_64 because the lock resolves torch==2.9.1, which has no wheel for this platform.