update example code to make ReferenceFileDocument sendable

latest Xcode (16) and associated SDKs have marked [ReferenceFileDocument](https://developer.apple.com/documentation/swiftui/referencefiledocument/) as Sendable. I inquired in the developer forums about this, and [the DTS engineer kindly explained](https://developer.apple.com/forums//thread/765302?answerId=807076022#807076022) that it has to be Sendable - which means adding some structure in there - because the various methods are called from a number of different isolation domains.

The gist is that the class holds ~3-4 properties that are all possibly mutating, and all of those need to be locked down to Sendable, so wrapped within an [OSUnfairAllocatedLock](https://developer.apple.com/documentation/os/osallocatedunfairlock) (as per updated [ReferenceFileDocument](https://developer.apple.com/documentation/swiftui/referencefiledocument/) code example).