Function audit 

pub fn audit(mat: &Material, masses: &impl MassProvider) -> Vec<AuditIssue>

Validate mass-fraction normalization, mass signs, key uniqueness, and atomic-mass availability.

• AuditKind::FractionsDontSum: the total mass is not positive-finite (empty, zero, NaN, or infinite total, so no fractions exist) or the normalized fractions sum outside 1.0 ± 1e-9.
• AuditKind::NegativeMass: one issue per nuclide with a negative or NaN stored mass.
• AuditKind::DuplicateNuclide: defensive only — comp is keyed by NuclideId, so duplicate keys cannot occur in memory; the variant exists for file/FFI consumers sharing AuditKind.
• AuditKind::UnknownMass: one issue per nuclide missing from masses.