Remove CPS tax-unit helper fallback after policyengine-us release

[MaxGhenis]

Context

#824 added policyengine_us_data/datasets/cps/tax_unit_rule_helpers.py, which imports shared CPS tax-unit helper rules from policyengine_us.tools.tax_unit_construction when available. It also keeps a temporary compatibility fallback so policyengine-us-data can run against the currently released policyengine-us dependency.

#8152 added the shared helper module to policyengine-us.

Work

After a policyengine-us release includes policyengine_us.tools.tax_unit_construction:

• Bump the minimum policyengine-us dependency in policyengine-us-data to that release.
• Remove the compatibility fallback definitions from tax_unit_rule_helpers.py.
• Keep the import surface in us-data thin so CPS tax-unit rules continue to live in policyengine-us.
• Run the CPS tax-unit unit/integration tests and the validation harness smoke checks.

Why

This avoids duplicated tax-unit rule logic across repos while preserving compatibility until the dependency release is available.