The Foundry could make stems (yt-dlp → demucs) but not eat stems somebody else
already made. Fred again's dropbox is 9 tracks × 6-13 producer-labelled stems at
96 kHz/24-bit — better source material than demucs can produce, and completely
outside what find_takes accepts: it keys stems by drums/bass/other/vocals, a dict
that collides on KEYS1+KEYS2 and an export keep= filter that would have silently
dropped every stem in the pack.

engine/stempack.py is the missing batch driver (TODO #20), and building it turned
up a real bug. MAREA ships its tempo in the filename (…123BPM…), so it is free
ground truth. At native 96 kHz the finder returned 123/123/124/119/128.1/128.5 bpm
and a 0.661 top score; resampled to 44.1 kHz first it returned 123.0-123.1 on every
candidate and 0.859, in a third of the time. librosa's hop/window defaults are
sample-rate-relative, so at 96 k every analysis frame spans half the musical time
and beat tracking wanders. Rate is a correctness input, not a performance knob.

engine/roles.py maps the pack's ~40 role tokens (39/40 hit directly) to a family,
then verifies the label by measurement before anything trusts it — the house rule
that a sound's role is never read off its name. The `hits` family
(KICK, SNARE alone) is the one place the label carries information the cheap features cannot: an isolated kick
and a full groove measure identically.

engine/kitcheck.py audits what grade.py cannot see by construction. A four-bar loop
whose last two bars are a fade grades S — silent-to-silent is a perfect seam and the
length is still exact. So: per-bar RMS for dead bars, onset-envelope autocorrelation
for whether a bar-aligned window is actually a repeating unit, and mel-fingerprint
cross-correlation for near-duplicates, because a pack shipping ALL DRUMS + KIT +
MAIN DRUM LOOP + DRUM BREAKS will otherwise ship one groove four times.

--jobs fans out as subprocesses, not a ProcessPoolExecutor: the pool's forkserver is
broken under this Python (BrokenProcessPool on a trivial task), and one process per
track also means a track that blows up loses only itself.

15 new tests.