Function free_space_path_loss_db 

pub fn free_space_path_loss_db(distance_km: f64, freq_hz: f64) -> f64

Free-space path loss in dB.

FSPL = 20·log₁₀(d) + 20·log₁₀(f) + 20·log₁₀(4π/c)

where d is distance in meters, f is frequency in Hz. This follows the standard telecommunications formula.