find_edges gains the same where/order_by/descending semantics find_vertices
received in 1.2.0, and count_edges joins count_vertices. The motivating
consumer is a bi-temporal relation store whose t_created/t_expired stamps
live on edge payloads: change-since queries need indexed text-range
comparisons over edges, not just vertices. create_payload_index already
works on edge tables unchanged.

Ten new tests cover edge text ranges with ordering, is_null/not_null on
expiry stamps, counting with relation_type composition, numeric predicates,
and index idempotence on an edge table, through both clients. 624 pass.