bug: Layers not marked as archived that are not part of any layer group don't get auto archived on state creation

If you define a layer without `archived: "true"` while using custom layout that doesn't display the layer, the layer is dropped

example:

```json
{
"layers": [
{
"name": "test",
"type": "image",
"source": "precomputed://gs://neuroglancer-public-data/flyem_fib-25/image"
}
],
"layout": {
"type": "row",
"children": [
{
"type": "viewer",
"layers": [],
"layout": "xy"
}
]
}
}
```

_Originally posted by @chrisj in https://github.com/google/neuroglancer/issues/929#issuecomment-4155514393_