Resolve conflicting tax unit role flags

[MaxGhenis]

Summary

• resolve imputed tax-unit role flag conflicts before constructing PolicyEngine tax units
• use household relationship hints so spouse/dependent conflicts do not create extra standalone filer units
• add regressions for head/spouse, head/dependent, and spouse/dependent conflicts

Validation

• uv run --python 3.13 --extra dev ruff check src/microplex_us/pipelines/us.py tests/pipelines/test_us.py
• uv run --python 3.13 --extra dev pytest tests/pipelines/test_us.py -k 'tax_unit_role_flags or role_conflicts'\n- structural probe on the current 100k-household candidate: tax units fall from 177,692 to 171,058; still above eCPS and needs broader coherence work\n