pulsar-four-beam-model: Geometry-first four-beam modelling code for pulsar phaseograms

Reference implementation of the four-beam, geometry-first model used in A common four-beam geometry reveals altitude-stratified GeV pulses in canonical young pulsarsPaul K. H. Yeung and Takayuki Saito, accepted by The Astrophysical Journal. The model decomposes Fermi-LAT gamma-ray phaseograms into two antipodal beam pairs: Beams 1 and 2: lower-altitude pair, close to the light-cylinder boundary; Beams 3 and 4: higher-altitude pair, contributing bridge/shoulder and ripple-like emission. The code is intentionally mechanism-agnostic. It parametrizes emission-site locations, bulk-flow directions, beam-axis orientations, phase-dependent Doppler shifts, and a compact generalized-normal-distribution beam kernel.