DESIGN SETUP (and office-hours' visual exploration) printed DESIGN_READY for
any executable binary, so one SIGKILLed at launch by an invalid macOS code
signature still passed. Launch it under the portable 10-second deadline
(gtimeout, timeout, or perl alarm); otherwise print DESIGN_NOT_AVAILABLE with
the exit code, 'timed out after 10s', or on 137 the code-signature hint and
'Fix: cd <checkout> && ./setup'. A success is cached by inode and mtime under
the state root so Gatekeeper's first-launch scan is paid once.
The probe launches the binary with no arguments (usage, exit 0): the design
CLI has no --version command today.