Directly addresses the #7 finding: per-stem PLP grids are weak on sparse-onset stems
(Doors/vocals recall 0.06 vs Gil-Scott-Heron 0.56), because PLP needs onsets to lock a
pulse and a vocal stem barely has them. Fix: find_takes now computes ONE beat grid from
the most rhythmic stem (drums → bass → first available) and reuses it for every stem, so
vocals/other are cut on the same musical bar lines as the drums instead of each guessing
its own. analyze_stem gained an optional `grid=(peaks,times,bpm)` param (None ⇒ per-stem,
the CLI case); find_takes and build_finder_eval both pass the shared grid.

Verified on Loituma: "beat grid from drums" → all four stems analysed on it, including a
cross-stem bass+vocals take at 137s. 8 finder tests (added: analyze_stem honours a shared
grid and adopts its tempo). build_finder_eval re-run with per-track drums grid is in
flight to quantify the recall lift over the 0.27 baseline; the correctness win (shared
grid ≫ weak per-stem grid on sparse stems) holds regardless. Pt.2 (sub-bar chop mode)
remains on #18.