Implement a Dependency Resolver (Topological Sort)

Question

## Problem

Given image/build-layer dependencies, return a valid build order, detecting cycles.

## Example
```
deps: A->B, B->C => order C,B,A; cycle -> error
```

## Constraints
- Up to 10^4 nodes

## Approach
Kahn's algorithm; report the cycle on failure. Relevant to image layer builds.