Review round 2 found no new defects; this is the one durable item it
asked to be carried further than a commit message.
`cmd | tail -2` returns TAIL's exit status, not `cmd`'s, in fish and
bash alike. So a gate chain of `cargo test ... | tail -2 && ... && echo
"ALL GATES CLEAN"` prints the clean line even when a suite failed, and
that is what happened while gating this lane: a `pmacs-gpu` failure was
summarized as clean. The point worth keeping is that this is not
carelessness a closer read would catch --- the verdict is structurally
absent from the summary the PR then cites. §5 now says to check
`$pipestatus[1]`, or better to redirect each gate to a file and read it
afterwards, which also preserves the full log that section already asks
for. Filed beside the skip-reports-`ok` lesson, which is the same
family: the thing that summarizes a gate must not be able to lose the
gate's verdict.
Also fixes the doc-comment splice in `tests/support/mod.rs`, where the
why-two-directories paragraph landed mid-sentence and left the
include-mechanics explanation stranded inside it. Cosmetic, and review
called it not worth a round on its own --- folded in here because the
file was being touched anyway.