-----Description----- 
This task requires writing a Lean 4 method that extracts even numbers from an array of integers. The method should return a new array containing only the even numbers found in the input array, while preserving the order in which they appear.

-----Input-----
The input consists of:
arr: An array of integers.

-----Output-----
The output is an array of integers:
Returns an array containing all the even numbers from the input array. Specifically:
- Every element in the output array is an even integer.
- All even integers present in the input array are included in the output array.
- The relative order of the even integers is preserved as in the input array.

-----Note-----
There are no preconditions for this task; the method will work with any array, including empty arrays (which are not null).