Practical: Flatten / Transform a Nested Block Tree

Question

## Problem

Notion docs are trees of blocks. Given a nested block structure, flatten it to an ordered list (with depth), or move a block subtree to a new parent preserving order. Practical, not LeetCode-style.

## Example
```
blocks tree -> ordered [(block, depth)]; move(subtree, newParent)
```

## Constraints
- Arbitrary nesting; preserve sibling order

## What Notion looks for
Practical coding in JS/TS over a real data model. Handle edge cases (move into own descendant).