not_livable_permits -- loading notes
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Rows: 135,159. Files: not_livable_permits.csv / .parquet / data_dictionary.csv

PREFERRED -- parquet (dtypes embedded, no re-contamination):
    import pandas as pd; df = pd.read_parquet('not_livable_permits.parquet')

CSV: read code columns as str (else '06037'->int 6037, leading zero lost):
    STR_COLS = ['permit_id', 'city', 'state', 'source_dataset', 'tract_geoid', 'county_fips']
    df = pd.read_csv('not_livable_permits.csv', dtype={c:str for c in STR_COLS})
FIPS canonical: state=2, county_fips=5, tract_geoid=11 digits.
Tight headline = uninhabitable_on_census_day & residential_flag & ~excluded_any (=32,199).