Full geocentric position: the six components Swiss Ephemeris returns from
swe_calc_ut(..., SEFLG_SPEED) plus a retrograde flag. A NEW struct (not an
extension of XalenPosition) so the existing xalen_planet_position ABI is
untouched. Speeds are daily motion — degrees/day for longitude/latitude,
AU/day for distance. is_retrograde is 1 when the (tropical) longitude rate
is negative, else 0.
Compute the FULL geocentric position of a planet: the six components Swiss
Ephemeris returns from swe_calc_ut(..., SEFLG_SPEED) plus a retrograde flag.
This is the high-fidelity counterpart to xalen_planet_position, which
fills only longitude/latitude/distance.