Count bills by status, type, source or period

How many bills fall in each bucket of one dimension, for any filter list_bills accepts. The answer to '¿cuántos se aprobaron?', '¿cuántos por tipo?', '¿cómo se reparten los del partido X?' — questions that otherwise mean paging every matching bill and tallying by hand (4,088 rows for one Colombian legislature, 50 at a time).

by is one of: status (where each bill currently stands — the latest tramitación's state), type (ley ordinaria, acto legislativo…), source (which body the record came from), legislative_period.

⚠️ source is where Dapper ingested the record, NOT the bill's constitutional origin. It does not reliably separate government bills from congressional ones; say so instead of inferring it.

by=origin is the one that does separate them, and only in Chile: Moción (congressional) vs Mensaje (government) is recorded for 100% of Chilean bills. The other eleven countries do not model it, so their bills land in the null bucket — a coverage gap, not 'origin unknown'.

Buckets with a null key are bills with no value recorded for that dimension — a coverage gap, not a category.