Recompute Dependent Formulas (Topological)

Question

## Problem

When a referenced value changes, recompute dependent formula cells in correct order; detect circular references.

## Example
```
A1 changes -> recompute B1=A1+1, C1=B1*2 in order
```

## Constraints
- Detect cycles

## Approach
Dependency DAG + topological recompute. Core to Coda's doc-as-app model.