Exact-motion solver controls

All six controls passed.

These tests use independently calculated receiver positions for constant northward motion on the declared disc metric, with gravity balanced by specific force. Two starting points are tested at interior, near-boundary and exact-boundary optima. The wider kernel uses the unchanged actual-position solver and numerical tolerances.

Only initial north position and north velocity vary. These are small numerical controls, not qualification of every nuisance direction, empirical calibration, or new Earth-model campaigns. There are zero archived-data fit starts. Original field/shape dispositions remain unchanged.