Closes the structural gap between bench/run.sh (one-shot capture
into JOURNAL) and a continuous tripwire. baseline.json records 31
metrics (16 throughput, 15 latency) with per-metric one-sided
tolerances tuned to absorb run-to-run noise on a quiet developer
machine; check.py spawns run.sh, parses both the throughput
pipe-table and the per-arm latency stanzas, diffs against the
baseline, prints a per-metric report, and exits non-zero on any
regression beyond tolerance.
User-facing flags: --from-file, --stdin, --baseline, and
--update-baseline (re-run + overwrite baseline.json after
intentional improvements).
Validation: captured the baseline, then re-ran end-to-end. All 31
metrics within tolerance. The harness also caught a single-capture
explicit-rc p99 spike (357.5 us) that was first read as drift vs.
yesterday's 18g.tidy.fu2 numbers but came in at 294.6 us on the
follow-up run — exactly the kind of noise the tolerance band is
there to absorb. Without 21'a we would have either chased a
phantom or buried the signal; with it, single noisy runs are data
points, not verdicts.
Tidy-iter discipline addition (run check.py at every family close
alongside the architect drift report) is recommended in JOURNAL
but not enacted in this iter — that's an orchestrator-level
update to CLAUDE.md.
