Evaluate Piecewise Linear Function

Given a list of 2D points [(x1, y1), (x2, y2), ..., (xn, yn)], where the points are ordered by strictly increasing x, connect each consecutive pair of points with a straight line segment to form a piecewise linear function.

Write a function that returns the function value at a target x.

Assumptions:

• target_x is guaranteed to lie within the range [x1, xn] .
• If target_x is exactly equal to one of the input x coordinates, return the corresponding y value.
• Otherwise, find the segment that contains target_x and use linear interpolation to compute the result.

Your function should take:

• points : a list of (x, y) pairs
• target_x : a numeric value

and return:

• the interpolated y value at target_x