retainedTargetCount
Reports active, retired, asynchronously releasing, and failed target-set permits across all registered devices.
Each physical device still enforces its own 64-set limit; this diagnostic sum does not combine their allocation budgets. It observes owner-thread accounting without allocating, polling, or retaining native resources and returns zero for an empty registry.
Return
the total permits still owned by registered devices until acknowledged physical destruction.
Throws
when called off the registry's owner thread.
when the aggregate cannot be represented as an Int.