Speed up ACS tax unit link inference

[MaxGhenis]

Summary

• Replace repeated per-household Pandas indexing in ACS spouse/parent inference with positional NumPy arrays
• Keep the existing spouse, parent, and imputation rules unchanged

Tests

• uv run ruff format --check policyengine_us_data/datasets/acs/acs_to_cps_columns.py
• uv run ruff check policyengine_us_data/datasets/acs/acs_to_cps_columns.py
• uv run python -m pytest tests/unit/datasets/test_acs_tax_unit_construction.py -q
• git diff --check