Remove obsolete S-curve functions and improve displacement integration

fixed bug #3720 and #3719
- Remove unused functions: getPhase, getTargetV, getNextPoint, getStoppingDist, findSCurveVSpeedConservative
- Rename velocity() to delta_velocity() and acceleration() to delta_accel() for clarity
- Replace analytical S-curve displacement with trapezoidal integration for stability
- Add trapz_distance() helper function
- Add currentjerk field to TC_STRUCT

Author: Luca Toniolo