BST Trim to Value Window

Given the `root` of a **Binary Search Tree** and two integers `loBound` and `hiBound` (where `loBound <= hiBound`), **trim** the tree so that all node values lie within the closed range `[loBound, hiBound]`. Return the root of the trimmed tree.

The trimming must preserve the BST structure. Nodes outside the range should be removed, and their children that remain in range should be re-attached to the tree correctly.

**Example 1:**
```
5
/ \
2 8
/ \
1 3
```
Input: root = [5, 2, 8, 1, 3], loBound = 2, hiBound = 6
Output: [5, 2, null, null, 3]
Explanation: Node 8 is removed (8 > 6). Node 1 is removed (1 < 2).

**Example 2:**
```
9
/ \
4 13
/ \
2 7
```
Input: root = [9, 4, 13, 2, 7], loBound = 4, hiBound = 9
Output: [9, 4, null, null, 7]
Explanation: Node 13 is removed (13 > 9). Node 2 is removed (2 < 4).