List a node's GPU units, so you can reserve or release them

The allocatable GPU units on one node, with the ids that create, release, and reclaim take. Without this those calls are unusable: they are unit-scoped by design, and nothing else on the API returns a unit id, so the first reservation on a node could not be written at all. Each unit is one IOMMU group and is indivisible: a paired unit sells its GPUs together. Only vfio-group units are reservable, and the response says which, and why not, rather than leaving the caller to infer it from a rejection. Readable by a platform operator, the org that owns the node, and any org managing a reservation on it. Anyone else gets 404, so the fleet stays unenumerable.