Chasing "does a shipped loop actually start on its downbeat" and nearly filing a false
alarm on the way. First measurement said 17% — but it scored 16 slots with no full-band
term while the rotator scores bars*4 slots WITH one, so the disagreement was the
instrument's. Factored the scorer out (_downbeat_slot_scores) so the rotation and its
verification cannot use lookalike measures, and re-measured with the real one.

The bug is real and smaller than the first number: over 60 shipped bar-loops, slot 0 was
the best slot in 29 and in the WORSE HALF of slots in 10. Median miss was 0.032 — i.e.
usually a near-tie — but one loop in six genuinely opens on a weak beat, which is exactly
the "timed ok but cut BCDA/DABC" complaint from 2026-07-11 that this function was written
to fix.

Mechanism: the roll point is zero-crossing snapped by up to ~10 ms against a 20 ms attack
window, and between two near-tied slots that is enough to flip which one wins. On clean
synthetic material rotation is idempotent (pass 1 finds slot 4, pass 2 finds 0); on real
music the tie-breaking is fragile.

So it now checks its own result and repeats, up to 3 passes, keeping a pass only if the
measured miss improves. Safe because rotation is free: two rolls compose into one and the
guard cannot oscillate. Best slot 29 -> 46 of 60, worse half 10 -> 1, median miss
0.032 -> 0.000.

Default is 3 rather than opt-in: it is strictly better and applies to every caller,
including export_take, so the GUI's forge gets it too. 112 tests.