Flatten a Multilevel Linked List

You are given a linked list where each node has three fields: `val`, `next` (pointer to the next node), and `child` (pointer to the head of a sub-list). The `child` pointer may or may not be `null`. Sub-lists can themselves have child pointers — creating multiple levels of nesting.

Flatten the list into a single-level doubly linked list in **depth-first order**: whenever a node has a child, the child list is fully inserted between that node and its `next` node, before continuing with the parent list.

Return the head of the flattened list.

**Example 1:**
```
Input:
Level 1: 1 <-> 2 <-> 3 <-> 4
|
Level 2: 5 <-> 6
|
Level 3: 7

Output: 1 <-> 2 <-> 5 <-> 7 <-> 6 <-> 3 <-> 4
```

**Example 2:**
```
Input:
Level 1: 10 <-> 20 <-> 30
|
Level 2: 40 <-> 50

Output: 10 <-> 40 <-> 50 <-> 20 <-> 30
```