feat(MM-65): add nix build output for relay binary using crane
- Add packages.relay output to flake.nix using crane's two-phase build
(buildDepsOnly → buildPackage) for cached dependency derivations
- Add crane input; pin via flake.lock
- Pass sqlite + pkg-config + LIBSQLITE3_SYS_USE_PKG_CONFIG=1 so rusqlite
links correctly, mirroring the devenv setup
- Set pname = "relay" to silence workspace Cargo.toml name warning
- packages.default = relay so bare `nix build` works too
Closes MM-65