flake: classify-paths large-input case fails with want == got #559

What happened

make ci failed once on the classify-paths test step:

✗ classify-paths test
      ok   grep-failure       aborts instead of answering
      FAIL large-input        want exit 0 + code=false/docs=true, got exit 0: code=false docs=true
    1 case(s) failed
make[1]: *** [Makefile:438: test-classify-paths] Error 1

Note the reported want and got are identical. The exit code was 0 and
the classifier's output was exactly what the case wants.

Why the message can't be trusted to explain it

scripts/classify-paths.test.sh (the large-input case) asserts three things
in one && chain:

if [ "$big_rc" -eq 0 ] && printf '%s' "$big_out" | grep -qx 'docs=true' && printf '%s' "$big_out" | grep -qx 'code=false'; then

but the failure branch reports only $big_rc and $big_out. So when the chain
fails for any reason other than those two values — either grep pipeline
returning non-zero under set -o pipefail, for instance — the message prints
a want/got pair that looks like a match and names nothing that differs.
That is exactly what happened here, and it is why this report cannot say what
the underlying cause was.

Reproduction attempts — none succeeded

• 40 serial runs of scripts/classify-paths.test.sh: 0 failures.
• 42 runs at 14-way parallelism (matching make verify's -j 14): 0 failures.
• A pipefail + SIGPIPE interaction was the obvious first hypothesis, since
  that is the bug fix(ci): make classify-paths fail loudly when grep fails #549 fixed inside classify-paths.sh itself. It does not
  apply here: grep -qx 'docs=true' matches only the last line, so grep must
  consume all input and never exits early. Verified directly at 200 KB and at
  the real ~20-byte payload — both pipelines return 0.

So the mechanism is unknown, and the flake is rare.

Ask

Make the assertion self-diagnosing, so the next occurrence explains itself
rather than costing another investigation:

1. Split the three conditions and report which one failed.
2. Print $big_out unambiguously (delimited or through od -c), not through
   tr '\n' ' ' — the current transform makes a single-line output and a
   correct two-line output print identically, which is one of the few
   explanations the evidence cannot rule out.
3. Consider grep -qx … <<<"$big_out" instead of the printf | grep pipeline,
   which removes the pipeline exit status from the assertion entirely. Cheap,
   and it eliminates a whole class of cause whether or not it was this one.

Provenance

Introduced with the classify-paths behavioral test in #549. Observed while
gating stack/3-content-type (#552); a re-run passed. Filing rather than
guessing at a fix, since the cause is not established.