BackgroundWorkerFactory

A user-supplied factory that builds BackgroundWorkers for a set of task ids, resolving the concrete worker lazily at dispatch time.

This is the bulk alternative to per-id registration (WorkerRegistry.register / BackgroundTaskManager.register taking a single task id + closure). Register one factory that owns many ids — typically one factory per app module, closing over that module's DI graph.

The taskIds / create sync contract. taskIds must enumerate every id create is able to build. The library leans on taskIds at start() to register OS-level handlers — on iOS each id is handed to BGTaskScheduler.register(forTaskWithIdentifier:), and every id there must also appear in the app's BGTaskSchedulerPermittedIdentifiers Info.plist array. An id that create can build but that is missing from taskIds will never get an OS handler and its scheduled work will silently never fire. Keeping the two in sync is the client's responsibility.

Conversely, if create is called with an id that is in taskIds but returns null, that's treated as a client bug — the registry throws WorkerRegistry.FactoryDeclinedException rather than falling through. create returning null is only legitimate for an id the factory does not own (the registry never calls it for such ids, but the nullable return keeps the contract explicit and chain-friendly).

Resolution order when multiple registrations exist: exact per-id registrations win first, then factories in registration order. Declared id sets must not overlap with each other or with per-id registrations — the registry rejects overlapping registration to keep resolution unambiguous.

@OptIn(ExperimentalObjCName::class): Swift-rename annotation for boundary refinement. Stable in practice and required by SKIE (CLAUDE.md §8).