Generic asset download, verification, and caching primitives.

This crate provides the building blocks used by arcbox-boot (VM boot assets) and arcbox-docker-tools (Docker CLI binaries) for downloading files from the network, verifying SHA-256 checksums, and writing them atomically to a local cache.

Feature flags

• download — enables the async download functions (pulls in reqwest, sha2, tokio, futures-util). Without this feature only the type definitions (progress, error, arch) are available.