Task Hierarchy: Subtask Roll-Up

Question

## Problem

Tasks form a tree (task -> subtasks). Compute completion percentage of a parent as the aggregate of its subtree, and find all overdue tasks.

## Example
```
parent done% = weighted subtree completion
```

## Constraints
- Arbitrary nesting

## Approach
Post-order DFS aggregating subtree state. Core to ClickUp's task model.