Minimum Swaps to Group Evens

Given a list of integers, determine the minimum number of adjacent swaps needed to group all even numbers together in one contiguous block.

Return the minimum number of adjacent swaps required.

Examples:

1. Input: nums = [3, 2, 4, 1, 6]
   Output: 1
   Explanation: The even numbers are at positions 1, 2, and 4. One adjacent swap is enough to make them consecutive.

2. Input: nums = [1, 2, 3, 4, 5, 6]
   Output: 2
   Explanation: The even numbers can be grouped into a contiguous block with two adjacent swaps.

Constraints:

• 1 <= len(nums) <= 10^5
• -10^9 <= nums[i] <= 10^9
• The input list always contains at least one even number.

Your solution should run in O(n) time.