- Closes apple#884. 

## Type of Change
- [ ] Bug fix
- [x] New feature  
- [ ] Breaking change
- [ ] Documentation update

## Motivation and Context
This PR implements the `container system df` command to display disk
usage statistics for images, containers, and volumes, along with their
total count, active count, size, and reclaimable space for each resource
type.

Active resources are determined by container mount references and
running state, while reclaimable space is calculated from inactive or
stopped resources.

Example output:
```
~/container ❯ container system df
TYPE           TOTAL  ACTIVE  SIZE      RECLAIMABLE
Images         4      3       4.42 GB   516.5 MB (11%)
Containers     4      2       2.69 GB   1.51 GB (56%)
Local Volumes  3      2       208.5 MB  66.2 MB (32%)
```

I'll have some follow-on PRs that will add `-v/--verbose` flag for
detailed per-resource information, `--filter` flag for filtering output
by resource type, and a `--debug` flag for debug statistics like block
usage, clone counts etc.

## Testing
- [x] Tested locally
- [x] Added/updated tests
- [ ] Added/updated docs