Add HCS (Host Compute Service) backend for Windows containers Add a new Windows-native backend using containerd for VHDX-based copy-on-write snapshots and container execution. New files: - lib/hcs_store.ml: Store implementation using containerd snapshots - lib/hcs_sandbox.ml: Sandbox using ctr run with OCI config - lib/hcs_fetch.ml: Base image fetcher using ctr image pull The store and sandbox are coupled through layerinfo.json files containing containerd snapshot keys and layer paths. Container networking uses HCN namespaces for NAT connectivity.