refactor(ClassicalMechanics): switch to derivVec for rigid-body velocities and momentum

[zhikaip]

Time.deriv sends Time → M to Time → M, so a trajectory of points previously had its velocity typed as a point. RigidBodyMotion.centerOfMassVelocity was Time → Space d, and linearMomentum inherited it, making momentum a position. This PR retypes them using derivVec (#1592), so velocities and momentum are valued in the displacement space EuclideanSpace ℝ (Fin d).

AI disclosure: assisted in checking and PR summary, code is written by myself

[github-actions]

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[morrison-daniel]

Looks good!