Function validate_truing_observations 

pub fn validate_truing_observations(
    observations: &[TruingObservation],
) -> Result<(), String>

Validate a truing observation set: every range finite and positive, every drop finite and non-zero, and no two observations at (numerically) the same range. run_multi_observation_truing_core runs this itself, so callers don’t have to — it is public so a front end can pre-validate when it wants to sequence its own progress output strictly after validation (the native CLI prints its “Fitting N observations…” progress line only for sets that will actually be fitted). Error strings are the stable user-facing ones.