Measures, scalars and descriptions

xtalforge_core::measure provides distance, angle, torsion, centroid and interplanar_angle. All atom-based functions expand the structure to P1 and use minimum images, so a bond across a cell boundary measures the same as one inside it.

use xtalforge::core::measure;

let d = measure::distance(&structure, 0, 1)?;
let a = measure::angle(&structure, 0, 1, 2)?;
let t = measure::torsion(&structure, 0, 1, 2, 3)?;

scalars exposes bond_lengths, coordination_numbers, atom_charges and atom_masses, driven by the perceived bond graph. describe_atom returns one line with element, label, site, coordination, fractional coordinates, charge and partial occupancy, used by tooltips, the CLI and the agent layer.

These are the first entries of the scalar registry; colouring and analysis name them instead of reimplementing the measurements.