Tree Boundary Perimeter Walk

Given the root of a binary tree, return all values of nodes on the **boundary** of the tree in counter-clockwise order, starting from the root.

The boundary consists of three parts, in order:
1. **Left boundary**: nodes from root to the leftmost leaf (excluding the leaf itself).
2. **Leaf nodes**: all leaf nodes from left to right.
3. **Right boundary**: nodes from the rightmost leaf up to the root (excluding the root and the leaf itself), listed in **reverse** (bottom-up) order.

Note: If the root has no left or right subtree, the root itself counts as a leaf.

**Example 1:**
```
Input: root = [1, 2, 3, 4, 5, 6, 7, null, null, 8, 9]
Output: [1, 2, 4, 8, 9, 5, 6, 7, 3]
Explanation:
- Left boundary (excl. leaf): [1, 2]
- Leaves (left to right): [4, 8, 9, 5, 6, 7]
- Right boundary reversed (excl. root+leaf): [3]
```

**Example 2:**
```
Input: root = [5, null, 8, null, null, 6, 9]
Output: [5, 6, 9, 8]
Explanation:
- Left boundary: [5] (root only, since no left child; root acts as left boundary start)
- Leaves: [6, 9]
- Right boundary reversed: [8]
```