Add a flake-parts flake whose default package Bun-compiles the kitchen
CLI into a self-contained native binary, built in the sandbox with
dependencies vendored by bun2nix.
Bun is pinned via a dedicated nixpkgs-bun input (Bun 1.3.13) that
bun2nix follows, so nixpkgs can track unstable without moving the
compile toolchain onto a release that emits empty binaries.
bun2nix is both a flake input and an npm devDependency, and a
postinstall hook keeps the checked-in bun.nix in sync with the lockfile.
