Preserve reverse sign in transient tire slip

[rhorvath02]

Finding

TransientSlip already stores signed longitudinal deformation: its -Vsx forcing term reverses with mirrored wheel kinematics, while the positive regularized speed magnitude provides stable relaxation in both travel directions. Multiplying u/sigma_kappa by an additional Vx_sign erased that reverse sign and made mirrored forward/reverse overspeed report the same longitudinal-slip sign.

Fix

Remove the redundant output sign multiplier and publish kappa = u/sigma_kappa (subject to the existing clamp). This matches the sign convention used by KinematicSlip while retaining positive, stable relaxation damping through sqrt(Vx^2 + V_min^2).

Validation

• The committed regression produced kappaForward = +0.0998922941, kappaReverse = -0.0998922941, and symmetry error 0 under CI's solver settings; all three values now have physical baselines.
• A wheel-speed step from pure rolling produced +0.0797510 forward and -0.0797510 reverse with zero symmetry error throughout the transient.
• At +/-0.1 m/s, regularized transient slip remained exactly mirrored at +/-0.0199623 and agreed in sign with KinematicSlip.
• The focused model matched all 31/31 initialization-baseline values.
• Current-main VehicleSim matched all 1018/1018 initialization-baseline values.
• A full VehicleSim run initialized without homotopy, terminated normally on the QSS plateau at 6.098631 s, retained only finite outputs, and kept minimum tire load positive at 182.83 N.
• The full forward maneuver retained the same QSS time and solver statistics as current main, as expected because the formulas agree for positive longitudinal speed.
• Ruff, Modelica lint, Python tests, and the complete OpenModelica translation sweep passed locally.