refactor(ClassicalMechanics): switch to derivVec for simple-pendulum velocity

[zhikaip]

S.spaceTrajectory θ : Time → Space 2 is the bob's position, a curve of points, so
differentiating it with Time.deriv gave a velocity typed as a point of Space 2 —
the docstring described it as "the vector tangent to the circle" while the type said
otherwise. This retypes it with derivVec (#1592), so the velocity is valued in
EuclideanSpace ℝ (Fin 2).

Nothing changes numerically: both norms are the L² norm on coordinates.

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

[github-actions]

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[morrison-daniel]

Looks good!