-----Description-----
This task requires writing a Lean 4 function that takes a list of stock prices and returns the maximum profit achievable by buying on one day and selling on a later day.

If no profit is possible, the function should return 0.

-----Input-----
The input consists of:
prices: A list of natural numbers representing stock prices on each day.

-----Output-----
The output is a natural number:
Returns the maximum profit achievable with one transaction (buy once, sell once), or 0 if no profitable transaction is possible.

