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

Replace the in-repo tax-unit construction engine and rule helpers with a
dependency on the standalone microunit package, re-pointing all call sites
(census_cps, the ACS wrapper, both validation scripts, and the tests). The
engine logic was extracted from this repo and is byte-identical; tax-unit
output is unchanged.

Delete policyengine_us_data/datasets/cps/tax_unit_construction.py and
tax_unit_rule_helpers.py. Keep the ACS column mapping and ACS wrapper, which
microunit deliberately excludes. Rewrite the CPS engine unit tests (which now
live in microunit) into thin integration tests for this repo's wiring; the
ACS integration tests are unchanged.

Pin microunit to a SHA until its first PyPI release and refresh uv.lock.

Fixes #1156

Co-Authored-By: Claude Opus 4.8 (1M context) <noreply@anthropic.com>

Author: Max Ghenis

changelog.d/1156.changed.md

@@ -0,0 +1 @@
+- Replaced the in-repo tax-unit construction engine and rule helpers with a dependency on the standalone `microunit` package, re-pointing all call sites. Tax-unit output is unchanged.

policyengine_us_data/datasets/acs/acs_to_cps_columns.py

@@ -1,6 +1,6 @@
 """
 Map ACS PUMS person records onto the CPS-like columns consumed by
-``policyengine_us_data.datasets.cps.tax_unit_construction``.
+``microunit.construct_tax_units``.
 
 Column contract:
 

policyengine_us_data/datasets/acs/tax_unit_construction.py

@@ -2,13 +2,11 @@
 
 import pandas as pd
 
+from microunit import POLICYENGINE_MODE, construct_tax_units
+
 from policyengine_us_data.datasets.acs.acs_to_cps_columns import (
     acs_person_to_cps_tax_unit_columns,
 )
-from policyengine_us_data.datasets.cps.tax_unit_construction import (
-    POLICYENGINE_MODE,
-    construct_tax_units,
-)
 
 
 def construct_tax_units_acs(

policyengine_us_data/datasets/cps/census_cps.py

@@ -5,9 +5,7 @@
 from zipfile import ZipFile
 import pandas as pd
 from policyengine_us_data.storage import STORAGE_FOLDER
-from policyengine_us_data.datasets.cps.tax_unit_construction import (
-    construct_tax_units,
-)
+from microunit import construct_tax_units
 
 
 OPTIONAL_PERSON_COLUMNS = {