Store stats as a system metadata file

[jqnatividad]

stats should produce a system metadata file, similar to how index creates an index file.

The stats metadata file will be stored in <FILENAME>.csv.stats similar to how index creates an index file at <FILENAME>.csv.idx.

When another command requires summary stats (schema, tojsonl, the WIP describegpt, etc.), it will check if the stats file exists and use that instead of compiling the stats anew.

[jqnatividad]

make it <FILENAME>.stats.csv so opening it with Excel and other tools will be easier as it will just be recognized as csv.

[jqnatividad]

Done with #902

[jqnatividad]

frequency, as of v0.130.0, now also uses the stats cache to get a column's cardinality to check if it has all unique values (i.e. row count == cardinality), and if so, short-circuit frequency table compilation for that column.