Dependency-Graph Evaluator for Reactive Expressions

Question

## Problem

Retool components reference each other via expressions. Given bindings, compute evaluation order, recompute dependents on change, and detect cycles.

## Example
```
B = A+1, C = B*2; A changes -> recompute B then C
```

## Constraints
- Detect circular references

## What Retool looks for
Reported Retool question. Build a dependency DAG, topological recompute, cycle detection — the heart of Retool's reactive engine.