Convert a numeric value between compatible units of measure. Supports: length (mm, cm, m, km, in, ft, yd, mi), mass (mcg, mg, g, kg, oz, lb, mt = metric tonne / 1000 kg, t = US short ton / 907.18 kg), volume (ml, cl, dl, l, kl, tsp, Tbs, fl-oz, cup, pnt, qt, gal, m3), temperature (C, F, K, R — non-linear conversions handled), speed (m/s, km/h, knot, ft/s), pressure (Pa, kPa, MPa, hPa, bar, torr, psi), energy (J, kJ, Wh, kWh, MWh), power (W, mW, kW, MW, GW), frequency (Hz, kHz, MHz, GHz), digital storage (b, Kb, Mb, Gb, Tb, B, KB, MB, GB, TB), and angle (deg, rad, grad). Incompatible units (e.g., km to kg) return an error identifying the quantity mismatch.