sharedInstance
Objective-C-visible accessor for shared. Swift callers use BackgroundTaskManager.shared, a bundled wrapper over this function; Kotlin callers use the shared property. Exists only because the property name collides with Kotlin/Native's generated companion accessor.
@OptIn(ExperimentalObjCName::class): Swift-rename annotation for boundary refinement. Stable in practice and required by SKIE (CLAUDE.md §8).