Short git commit hash the binary was built from, stamped by build.rs at
build time via pure std::fs (no shellout — the zero-shell law). env!
(not option_env!) because build.rs emits ZNIPPY_GIT_HASH on every
build; unknown off a git checkout.
The canonical version-identity line: v<CARGO_PKG_VERSION> (<git hash>).
The ONE source of truth for the znippy binary’s identity string — printed
by znippy --version / -V. Combined with clap’s command name znippy,
the version flag emits znippy v<version> (<hash>).