Code Explainer

Explain the following code to me. I'm a competent developer but I haven't worked with this codebase before.

Give me three layers:

**Layer 1: What it does**
One paragraph. What is the purpose of this code? What problem does it solve?

**Layer 2: How it works**
Walk through the main logic in order. Name the key steps. Point out anything non-obvious.

**Layer 3: What to watch out for**
- Are there hidden assumptions or preconditions?
- Are there known footguns or edge cases I should know about?
- Is anything here likely to cause confusion for someone new to the code?

After explaining, answer: if I wanted to change [specific thing], where would I start?

```
[paste code here]
```