Re-ran build_finder_eval with the shared drums-derived beat grid. Recall 0.27→0.34
(+26% rel), precision 0.24→0.31 (+29% rel) over the 8 most-cut stems. The lift is
concentrated exactly where predicted — vocal stems: Xxplosive/vocals 0.27→0.50,
Doors/other 0.27→0.37. Confirms the §7 diagnosis (weak per-stem PLP grids on
sparse-onset stems). Still below the 0.70 target; the remaining gap is sub-bar
chops the bar-aligned finder can't form — #18 pt.2 (onset-driven chop mode).