Reads the option cvmdata.source, falling back to "mirror"
(parquet via DuckDB) when the option is unset.
Details
From v0.1.0 onward the built-in default is "mirror": it ships the
full historical series for CAD, DFP, ITR and FRE as parquet assets
on GitHub Releases, refreshed weekly. The "cvm" backend, which
reads the CVM Open Data Portal directly, remains fully supported
and can be selected per call or persisted with cvm_source_set().
See also
Other source:
cvm_source_set()