Reads the per-year xpath counts and filing counts that build_evidence()
writes from the ef2 DuckDB builds (xpath_stats.csv and filings.csv),
for one or more evidence directories: evidence (990 and 990-EZ returns)
and evidence/pf (990-PF returns). These are the observations
xpath_version_fields() uses.
To refresh them from new or rebuilt ef2 databases, run build_evidence()
and combine_evidence() (see data-raw/build-evidence.R and
data-raw/build-evidence-pf.R), then update_xpath_versions().
In the ef2 builds the tax year of a filing is the year of its schema
version, so tax_year and the year of schema_version agree.