Reformulate the cost difference calculation (`ellipticCostDif`) to use mathematically equivalent formulas that avoid subtracting large, nearly equal values (cancellation errors) in single precision at high normal forces.
This is a C port of Alain's formulation in MJWarp:
https://github.com/google-deepmind/mujoco_warp/pull/1512
Also adds an integration test (`EllipticLineSearchPrecisionDiagnostics`) that reproduces the precision issue under large normal forces in the sliding regime, and asserts that the solver does not produce large negative improvements in either precision. This test failed before the change.
PiperOrigin-RevId: 946137815
Change-Id: Ia8fc1c4823b5fee770140c8989b9465737d22ad7
Yuval Tassa
892d889793