A misconfigured environment (wine missing, msitools not installed, a
partially-built toolchain) otherwise only surfaces as a wine-specific
error buried deep inside a build. `vintner doctor` checks wine itself
(found and actually runs), the optional extraction tools download
needs, and every installed <dest>/bin/<arch> toolchain's on-disk
layout, printing a pass/fail checklist and exiting non-zero if
anything's broken.
