build_tierlist.py grades EVERY hand-cut loop in the Samples corpus with the Foundry
katana (engine/grade.py — DRY, one rubric) and joins each kit against how often PLN
actually plays it (`s "kit"` track-counts from catalog_view.json). 1690 wavs across
97 loop-kits graded in parallel (~98s, 6 workers). Tiers: 4 S, 3 A, 24 B, 51 C, 15 D.

The point of #11 was never the ranking — it's VALIDATING the rubric against reality
(PHASE2_FINDINGS §5b): do the kits PLN reaches for grade higher? Result:
grade↔usage Spearman ρ = +0.34 — positive, the katana points the right way.

The confound is the interesting part. The single most-used kit, `risers` (16 tracks),
tiers only C — and that's CORRECT: risers are FX sweeps, they go quiet→loud and never
wrap cleanly, so they fail seam + bar-consistency by design. They're heavily used as
one-shot transitions, not loops. Same for weird_dialogs. So loop-mechanical-grade and
usage diverge exactly where they should — for non-loop material. Filtering FX out would
lift ρ; the divergence is a feature (it tells loops from FX), not a rubric failure.
Meanwhile PLN's custom `f*` palette validates the rubric from the other side: fsynth,
fguitar, fmono all land A-tier; fpiano/forgan/fbreak* B. S-tier exemplars: simmons,
electrn, samples-cello-plucked, ouais.

Caveat logged for follow-up: only 34/97 kits matched a usage token (folder name vs
`s`-token mismatch for the rest, and catalog_view covers 73 tracks) — the correlation
is over those 34. The grader's v1 thresholds (noisy dc-offset flag at 1e-4, seam dB
cutoffs) are the next calibration lever now that we can measure against the corpus.

7 pure-logic tests (spearman incl. ties/inverse, usage = tracks-not-occurrences,
tier thresholds mirror the grader). Next: tierlist.html showcase (Ship's Bridge + impeccable).