Level-Order Wave Sum

You are given the root of a binary tree. Perform a **level-order (BFS) traversal** and return the **sum of node values at each level** as an array, where the result array's `i`-th element is the total of all values on level `i` (0-indexed from the root).

**Example 1:**
```
Input: root = [5, 3, 8, 1, 4, 7, 9]

5
/ \
3 8
/ \ / \
1 4 7 9

Output: [5, 11, 21]
Explanation:
Level 0: 5 → sum = 5
Level 1: 3 + 8 → sum = 11
Level 2: 1 + 4 + 7 + 9 → sum = 21
```

**Example 2:**
```
Input: root = [1, 2, 3, null, 5]

1
/ \
2 3
\
5

Output: [1, 5, 5]
Explanation:
Level 0: 1
Level 1: 2 + 3 = 5
Level 2: 5
```