Complements the task-granular background_work with the async-delegation
UNIT count (each dispatch/batch = 1), recovering the pool-slot semantics
active_count() gives. Together: background_work = real concurrent subagent
load (batch expanded), background_delegations = slot pressure to alert
against delegation.max_concurrent_children. Registered in metric_names,
documented, and covered by a task-vs-unit contract test.
cc80dba96f