The LoonFS object-store boundary.
LoonFS assumes only the narrow provider contract the format spec names —
create-if-absent, compare-and-swap, read-after-write visibility, prefix
listing — and this crate owns that boundary: the [ObjectStore] trait,
provider adapters for S3, Cloudflare R2, Google Cloud Storage, Azure Blob
Storage, and the local filesystem, the durable key layout in [keys] and
[layout], and the contract probe in [probe] that proves a configured
store honours that contract — the same checks the conformance suite runs
against real providers.