Built-in binary cache server and push pipeline for sui.

Replaces Attic, Cachix, and nix-serve with a single integrated component. Implements the standard Nix binary cache HTTP protocol (narinfo + NAR).

Architecture

• [sui_castore] — pluggable storage backends (shared with sui-registry); re-exported from here for backward compatibility.
• [server] — axum HTTP server implementing the cache protocol
• [signing] — ed25519 key management and narinfo signing
• [push] — pipeline to push store paths to the cache
• [gc] — garbage collection of unreferenced cache entries
• [config] — cache configuration types (CacheConfig; BackendConfig is in sui-castore)