Problem — the finder's rank didn't reflect true seam quality. On a real
end-to-end run over the Super freak dub kit (Soul Sugar meets Dub Shepherds),
several top-scored 2-bar windows CLICKED at the wrap after export while cleaner
windows ranked below them. Root cause: analyze_stem scores the seam on the RAW
candidate window, but export_take zero-crossing-snaps both boundaries (B1) AND
slices at the 3-decimal-rounded start_s/end_s — and both transforms move the wrap.
A beat-exact window measuring seam 0.79 read 0.07 once rounded, and that rounded,
snapped slice is what actually ships. So the finder's seam proxy diverged from the
grader (grade.py), which measures the window that exists on disk — the ground truth.

Approach — a verify-rerank pass (loops._verify_rerank) after candidate generation,
before dedup/top_n. It re-measures the ~3×top_n survivors' seam on the TRUE
post-snap window — the exact zc-snapped, rounded-bounds slice export writes —
reusing grade.seam_score (DRY, no duplicated DSP). The proxy seam term is swapped
for the true seam in the composite (same weights), and any candidate whose true
seam falls below VERIFY_SEAM_FLOOR is hard-demoted so a click can never top the
list. Gated behind a new analyze_stem(..., verify=True) kwarg (default on; False
reproduces the pre-#19 raw-proxy ranking for autotune baselines). Public signatures
(analyze_stem, find_takes, weights dict) unchanged — autotune.py/server.py intact.

Also fixed a sparse-material seam false-positive in grade.seam_score: the wrap
curvature was normalized by the MEAN |2nd-diff|, which collapses to ~0 on sparse
dub percussion (mostly silence + a few hits), blowing the click ratio up (129× on
a genuinely clean loop). Now normalized by the 90th-percentile |2nd-diff|,
amplitude-floored — 2–3× on the same clean loop.

Validation (drums+other stems, bars 1/2/4, shared 132.5 BPM grid,
export-faithful grades):
  drums top-3  BEFORE  C 0.65 / C 0.63 / A 0.80   (clicks @93.08s, @24.06s)
               AFTER   S 0.98 / S 0.95 / A 0.81
  other top-3  BEFORE  B 0.72 / B 0.72 / B 0.70   (bad favorite @56.74s clicks)
               AFTER   S 1.00 / S 0.98 / S 0.99
The known-good drums 4-bar @107.21 (S) and other regions surface; the clicking
other @56.74 is demoted out of the top entirely.

Tests: +2 verify-rerank regression tests (a proxy-clean but discontinuous wrap that
survives the snap is demoted below a seamless one; verify=False leaves the proxy
untouched) and +1 sparse-percussion test (a clean sparse loop is NOT flagged
clicking). Suite 57 → 60 green.