HasObservationCount
Result trait for snapshots that know how much evidence they are built from.
Exists so a caller can ask "did this stat see anything?" without knowing which field carries the answer. The count is spelled totalWeights on most results, totalSeen on the sketches and totalWeight on a few others, so before this trait the emptiness check was per-stat knowledge.
The distinction matters because several results report a numerically plausible value on an empty stream. A quantile sketch with no observations reports NaN per quantile precisely so that absence is distinguishable, but a mean reports 0.0, which is not. Gate on isEmpty before trusting any field of a snapshot that might have arrived before the first observation did.