Calculate free-space path loss (FSPL) in decibels using the ITU-R P.525 standard formula. Given a frequency in MHz and distance in kilometres, returns the expected signal attenuation in an ideal line-of-sight radio link with no obstacles, reflections, or atmospheric absorption. Also computes the RF wavelength. Use this to estimate baseline path loss before adding fade margins, antenna gains, or environmental corrections. Feeds directly into link_budget for full end-to-end analysis.