Function copy

Copies sourcePath on source to destPath on sink. When sourcePath is a folder, recurses: uses a folder-aware directCopy when the source declares supportsRecursiveDirectTransfer, otherwise lists the folder and transfers each entry (bounded by options.concurrencyLimiter), following cp -r target semantics (see resolveRecursiveDestRoot).

For a single file: uses source.directCopy when source.canDirectTransfer?.(sink) reports eligibility. Otherwise falls back to multipart transfer (when both sides support it, size crosses options.multipartThreshold, and part-size negotiation succeeds) or plain streaming - retried per options.retry on TransientIOError.