Refinement caught before re-cutting the real kits: superfreak drums and
bass ship from the SAME window (107.21s, 4 bars). If each stem rotated
to its own strongest attack, their relative groove could shift by whole
beats — bass's phrase-start is not necessarily on the drums' 1, and the
kit is played together (loopAt on a common cycle), so relative phase
between kit loops is musical content.

export_take now derives the downbeat slot ONCE per shared time window
from its most rhythmic stem (drums > bass > other > vocals priority)
and applies that same slot to every stem of the window
(_rotate_to_downbeat force_slot; per-stem roll still zc-snaps within
±10 ms on the stem's own mono ref). Stems at different windows keep
independent rotation. Stem reads are cached (each stem was loaded twice
otherwise).

Test: two-stem synthetic window, drums downbeat at slot 3, other's own
loudest attack at slot 6 — drums' marker must land at 0 and other's at
3 (= 6−3, following drums), not 0. 86 passed.