Recompute Dependent Cells on Change

Question

## Problem

When cell B changes, recompute exactly the cells that (transitively) depend on B, in correct order, once each.

## Example
```
B changes -> recompute its dependents in topological order
```

## Constraints
- Avoid recomputing unaffected cells

## Approach
Reverse-dependency traversal from B + topological ordering. Airtable's formula-engine problem.