{session scope, send_id} - the key a cancel addresses.
Deliberately not unique: spec 6.3 cancels every delayed send under a
sendid, and an author-written id is reused verbatim without advancing
send_counter, so one <send id="x" delay="..."> executed twice stores
two rows under one key. A cancel matching nothing is a no-op, not an
error (ADR-0054 decision 3).
ADR-0059 leaves this key untouched where it amends the dedup key: spec 6.3's cancel-them-all semantics is exactly what the pair exists to express.