Two errors stacked in one metric, both found by looking at its distribution instead
of its verdicts.

**It penalised short loops for being short.** Autocorrelation at lag k has only N-k
overlapping frames, but the score was normalised by ac[0], which sums all N — so the
number was partly a function of how much of the loop the lag consumed, and a 1-bar lag
consumes half a 2-bar loop against an eighth of an 8-bar one. On a perfectly periodic
click track the biased estimator reports 0.418 / 0.623 / 0.754 at 2 / 4 / 8 bars for
IDENTICAL material. Dividing by the per-lag overlap count first: 0.834 / 0.830 / 0.862,
flat as it must be. Corpus medians moved 0.12 -> 0.24 (2 bar), 0.31 -> 0.41, 0.43 -> 0.49.

**And then it asked every role the same question.** Split by family across 81 bar-loops:
drums 0.754, fx 0.549, tonal 0.214, bass 0.173, vox 0.087 — an 8.7x spread end to end.
A drum loop that does not repeat at the bar is broken. A sung phrase that does not repeat
within itself is a sung phrase, and a held chord is a perfectly good loop with no onset
period at all. One threshold across all of them condemned 12 of 20 vocal loops for the
crime of being vocal. Floors are now per family, and vox is measured-but-never-flagged.

Also: dead bars are now rejected at EXPORT rather than reported afterwards. A 4-bar loop
whose last two bars are a fade grades S — silent-to-silent is a perfect seam and the
length is still an exact bar multiple — and 10 of the first 161 cuts were exactly that.
An objective defect with an objective test belongs in the gate, not in a report someone
has to read.

First full run for the record: 161 samples, 43 kits, ZERO bar-length errors and 161 of
161 distinct sounds, so the bar-multiple guarantee and the export dedup both hold at
scale. 21 tests in this file, 104 in the suite.