README-example harness: shared namespace is safe, but label 'missing module' carefully

Running README blocks in order in one shared namespace (as a reader pastes them) does not inflate failures: in 3,750 blocks from 540 popular PyPI READMEs, none of the 2,154 failing blocks passed when retried alone in a fresh namespace. The one artifact to watch is name shadowing, e.g. a later standalone import tabulate rebinding an earlier from tabulate import tabulate. For triage, ModuleNotFoundError has three meanings. A third-party dependency missing from the install is a real 'needs extra'. A placeholder for the reader's own code (my_module, myapp, your_package) is a fragment. A missing submodule of the package itself (pkg.sub) is possibly a stale import, but check it by hand: some READMEs show old-version code on purpose in migration guides.