Depend on microunit for tax-unit construction instead of in-repo copy

[MaxGhenis]

Fixes #1156

Summary

Replaces this repository's in-repo tax-unit construction engine and rule helpers with a dependency on the standalone microunit package, and re-points every call site. microunit is the canonical home for these rules going forward; the engine logic was extracted from this repo and is byte-identical.

This is a behavior-preserving refactor: tax-unit output is unchanged (verified — see below).

What changed

Deleted (logic now lives in microunit):

• policyengine_us_data/datasets/cps/tax_unit_construction.py — the engine.
• policyengine_us_data/datasets/cps/tax_unit_rule_helpers.py — the federal rule helpers.

Re-pointed to import from microunit:

• policyengine_us_data/datasets/cps/census_cps.py (construct_tax_units)
• policyengine_us_data/datasets/acs/tax_unit_construction.py — the ACS wrapper (POLICYENGINE_MODE, construct_tax_units)
• validation/cps_tax_unit_validation.py (POLICYENGINE_MODE, SUPPORTED_TAX_UNIT_CONSTRUCTION_MODES, CPSRelationshipCode, construct_tax_units, qualifying_child_age_test)
• validation/cps_tax_unit_outcome_validation.py (construct_tax_units)
• policyengine_us_data/datasets/acs/acs_to_cps_columns.py — docstring reference only.

Kept in this repo (deliberately excluded from microunit):

• policyengine_us_data/datasets/acs/acs_to_cps_columns.py — ACS-specific column mapping.
• policyengine_us_data/datasets/acs/tax_unit_construction.py — the thin ACS wrapper that maps ACS columns to CPS-like columns and adds the ACS imputed-link flags before delegating to microunit.construct_tax_units.

Dependency: added microunit @ git+https://github.com/PolicyEngine/microunit@d3eccbbd33aa51f1c310bd6c2f37c9c3735beeb1 to pyproject.toml (pinned to a SHA until the first PyPI release) and refreshed uv.lock.

Test coverage: moved vs kept

The CPS tax-unit construction engine unit tests are pure-engine and now live in microunit (its tests/test_tax_unit_construction.py is the same 22-test suite, function-for-function). To avoid maintaining those rule tests in two repos while keeping this repository's own integration paths covered, tests/unit/datasets/test_cps_tax_unit_construction.py is rewritten from a copy of the engine suite into thin integration tests that exercise this repo's use of microunit — CensusCPS._create_tax_unit_table wiring (construction-mode and dataset-year pass-through, constructed-TAX_ID write-back, and CENSUS_TAX_ID preservation).

The ACS integration tests (tests/unit/datasets/test_acs_tax_unit_construction.py) are unchanged: they still cover ACS column mapping and ACS.add_id_variables wiring, which remain in this repo.

Behavior preservation

The engine body (everything from the HEAD = "HEAD" marker onward) is byte-identical between the deleted module and microunit; only the helper-import lines differ.

Verified empirically against the pre-refactor engine extracted from main, under the pinned policyengine-us==1.715.3:

• dependent_gross_income_limit returns identical values for every year 2019–2026 (4,200 / 4,300 / 4,400 / 4,700 / 5,050 / 5,200 / 5,300).
• 3,000 comparisons across randomized households × {2022, 2024, 2026} × {policyengine, census_documented} produced 0 differences in either the person-assignment frame or the tax-unit frame.

Note for reviewers: the data source for the qualifying-relative gross-income limit changes — microunit vendors its own dependent_gross_income_limit.yaml rather than reading policyengine-us parameters at runtime — but the resolved values are identical for the pinned policyengine-us, so output does not change. This repository never shipped that YAML itself, so nothing was removed from its package data.

Checks

• make lint: clean.
• uv run pytest tests/unit/datasets/test_cps_tax_unit_construction.py tests/unit/datasets/test_acs_tax_unit_construction.py: 18 passed.
• uv run pytest tests/unit: 2044 passed, 4 skipped.