Maximum Average Subarray

Given an integer array nums and an integer k, find the subarray of length k with the maximum average value and return that maximum average.

Example:

Input:  nums = [1, 12, -5, -6, 50, 3], k = 4
Output: 12.75  (subarray [12, -5, -6, 50] → average = 51/4 = 12.75)

Constraints:

• 1 ≤ k ≤ nums.length ≤ 10⁵
• -10⁴ ≤ nums[i] ≤ 10⁴