gatelineAPI reference

The reason rule DL writes, and the key its resolution check matches on (#343).

Deliberately shaped so no other rule's matching claims it: D17 keys on task <id> and D20 on (<id>), so the task is named on <id> here and never parenthesized. One constant for both halves, as with VERIFIER_ESCALATION_KEY, so the writer and the reader cannot drift apart into perpetual re-escalation.