Decouple storage from the ZoneRuntime trait into a dedicated StorageEngine
trait with ZfsStorageEngine (illumos) and MockStorageEngine (testing)
implementations. Replace the per-zone ZfsConfig with a global
StoragePoolConfig that derives dataset hierarchy from a single --storage-pool
flag, with optional per-dataset overrides. This enables persistent volumes,
auto-created base datasets on startup, and a clean extension point for
future storage backends.
Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>
