Given two sorted arrays `nums1` and `nums2` of size `m` and `n` respectively, return the **median** of the two sorted arrays.
The overall run time complexity should be O(log(m+n)).
**Example 1:**
```
Input: nums1 = [1,3], nums2 = [2]
Output: 2.0
Explanation: merged = [1,2,3] → median is 2.0
```
**Example 2:**
```
Input: nums1 = [1,2], nums2 = [3,4]
Output: 2.5
Explanation: merged = [1,2,3,4] → median is (2+3)/2 = 2.5
```