Fixed InfrahubNode.download_file(skip_if_unchanged=True) on an unsaved node: previously, a node whose local dest happened to match the node's in-memory checksum.value could silently return 0 without a network round-trip. The method now enforces the saved-node check before the skip-check short-circuit, raising ValueError("Cannot download file for a node that hasn't been saved yet.") as it does in the non-skip path.