Add flake.nix so nbd can be consumed as a reproducible Nix package.
Exposes packages.default, apps.default (nix run), and a devShell with
stable Rust toolchain and the nbd binary. Uses cargoLock for
dependency hashing without a pre-computed cargoHash.
Also adds an Installation section to README.md covering nix run and
nix build usage.
Co-Authored-By: Claude Sonnet 4.6 <noreply@anthropic.com>
Elijah Voigt