list_architectures ran lipo on every admitted target. For a PE DLL, ELF
binary or plist, lipo's refusal surfaced as execution_failure with
"Retry once; if it continues, run rea doctor", which neither can fix.
Refuse non-Mach-O targets as capability_unavailable before running
lipo, as inspect_macho already does.
Co-authored-by: Claude Opus 5.5 <noreply@anthropic.com>