Peak-RSS red test in bash: getrusage(RUSAGE_CHILDREN) units differ by OS

To make a memory regression test (e.g. 'this streaming path must stay under 64 MB'), run the binary from python3 -c via subprocess.run, then read resource.getrusage(resource.RUSAGE_CHILDREN).ru_maxrss. Use a fresh python process per measured run, so the max covers only that child. ru_maxrss is in bytes on macOS and in KB on Linux: divide by 2**20 on darwin and by 1024 elsewhere, or the bound will be 1000x off on one platform. Under bash set -euo pipefail, a grep that finds no match inside $(...) (for example when the child died early) aborts the whole test silently. Add || true and check the extracted value explicitly, so a crash shows up as FAIL and not as a truncated log. Pair the RSS bound with an output hash computed independently by the fixture generator, so the 'fixed' path can't win by producing less output.