new op gather/merge flag: 'c' for cardinality

it just uses a simple cardinality estimator wrapped in a value
object ala DataCounting. Was actually surprisingly easy -- payoff
from some earlier changes to refactor MergeConfig I suppose.
new query op: "filter" for using any bundle filter

may want to use index-based fields by using eg. `from.index: 3`
promote bundle-filter `_default._primary: filter`