ClockDriftDetectingStream - needs to "open" clock existential

ClockDriftDetectingStream.swift

/// `some` opens the existential clock and instants
func clockDriftDetectingStream(
  _ clock: some Clock<Duration>,
  checkEverySeconds: Double,
  toleranceSeconds: Double
) -> AsyncStream<Void> {
  let startInstant  = clock.now
  let period        = Duration.seconds(checkEverySeconds)
  let tolerance     = Duration.seconds(toleranceSeconds)
  
  precondition(tolerance < period, "tolerance must be less than the period")
  
  let (stream, continuation) = AsyncStream<Void>.makeStream()
  let task = Task {
    var expectedInstant = startInstant
    
    while !Task.isCancelled {
      do {
        try await clock.sleep(for: period)
      } catch {
        break
      }
      expectedInstant = expectedInstant.advanced(by: period)
      let now = clock.now
      // ❌ This wouldn't compile without opening `clock`
      let drift = expectedInstant.duration(to: now)
      let negativeTolerance = Duration.zero - tolerance
      if drift > tolerance || drift < negativeTolerance {
        continuation.yield(())
      }
    }
    
    continuation.finish()
  }
  
  continuation.onTermination = { _ in
    task.cancel()
  }
  
  return stream
}