"""Computes the first-order approximation of a rotation matrix from a rotation vector.

The function constructs a 3x3 rotation matrix using the first-order terms of the rotation vector. The diagonal elements are set to 1.0, and the off-diagonal elements are derived from the components of the rotation vector.

Args:
    rotvec (np.ndarray): A 3-element rotation vector representing the rotation.

Returns:
    np.ndarray: A 3x3 rotation matrix as a first-order approximation.
"""