AstroLink propagation engine reaches v2

Our propagation core now integrates the complete force model we set out to build.

The engine numerically integrates atmospheric drag (JB2008 density with HWM14 winds), solar radiation pressure with eclipse transitions, luni-solar third-body gravity and the Earth's full spherical-harmonic geopotential, with native ECI/ECEF/TEME conversions, CSPICE-compatible.

This is the foundation every AstroLink product builds on.