Persistent store coordinator
Essence
NSPersistentStoreCoordinator (PSC) is the layer between contexts and physical stores. It knows the model (NSManagedObjectModel), holds the list of attached NSPersistentStores and translates context operations (fetch, save) into calls on a concrete store: SQL for SQLite, a file write for Binary, in-memory operations for In-Memory. A context doesn't know where the data lives — it talks only to the coordinator.
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