fix: show binary stderr on version check failure

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Summary

• include captured stderr when a downloaded binary fails its -V check
• preserve the existing status-only error when the child process emits no diagnostic
• cover loader errors with a regression test based on the macOS libusb failure

Context

The Foundry v1.8.1 Intel macOS binary aborts when its linked libusb dylib is unavailable. Command::output captured the useful dyld diagnostic, but get_bin_version discarded stderr and reported only SIGABRT, hiding the actionable cause.