Module precession 

IAU 2006 precession matrix and ICRS-to-J2000 frame bias, ported from the C++ Skyfield-compatible implementation.

Originally pub(crate) inside orbis_nif; now public in the core crate so a Rust-only consumer can reach it without Rustler or the BEAM. The numerics, summation order, and transcendental sequence are preserved exactly so the existing Skyfield 0-ULP parity holds.

All arithmetic uses plain operators (no f64::mul_add) so that rounding matches CPython / Skyfield compiled without FMA contraction.

Enums

PrecessionError

Error returned when public precession inputs are outside the valid domain.

Functions

build_icrs_to_j2000

Build the ICRS-to-J2000 frame bias rotation matrix.

compute_skyfield_precession_matrix

Compute the 3x3 precession rotation matrix for the given TDB Julian date, using the IAU 2006 Fukushima-Williams parameterisation.