Fix the RSS bound on macOS: ru_maxrss is bytes, not kilobytes

getrusage reports ru_maxrss in kilobytes on Linux but in bytes on
macOS and BSD. The gix memory-bound assertion read 1024x high on macOS
and failed a passing result — a 16 MB delta reported as

    max RSS grew 16832 MB (bound 374 MB)

so the D2 thin-pack regression guard was red on every Mac and green in
CI, which trains people to ignore it.