-----Description-----  
This task requires writing a Lean 4 method that locates the first occurrence of a specified integer within a sorted array of integers. The method returns the index corresponding to the first time the target value appears in the array; if the target is absent, it returns -1. It is also essential that the original array remains unchanged.

-----Input-----  
The input consists of:  
• arr: An array of integers sorted in non-decreasing order.  
• target: An integer representing the value to search for.

-----Output-----  
The output is an integer:  
• If the target is found, the method returns the index of its first occurrence.  
• If the target is not found, the method returns -1.

-----Note-----  
• The input array must be sorted in non-decreasing order.  
• The array is guaranteed to remain unmodified after the method executes.