The B1 zc-snap moved each loop edge independently (±10ms tolerance), so
exported loop DURATIONS drifted off the bar grid: measured −4.2ms on a
4-bar (superfreak bass) and −11.4ms on 2-bar loops (hammer kit, both
edges snapped inward). loopAt absorbs the drift, but the kit contract we
want is stronger: every exported loop's duration == bars × bar_len.

Fix — _snap_length_preserving(): hold the exact bar-multiple sample
count RIGID (bar_len_samples from the take's grid BPM) and slide the
WHOLE window by a single common offset δ ∈ ±ZC_TOL_MS, picking the δ
that minimizes combined edge amplitude |y[start]|+|y[end]| (tie-break:
smallest |δ|). Both edges move together ⇒ duration is preserved by
construction; guarantee |exported_dur − bars·bar_len| < 1ms (measured
<0.02ms on real stems). Chops (bars=0) keep the per-edge snap.

_snapped_window/_true_seam (verify-rerank, 791a0d78) now mirror the same
scheme so the finder still measures the exact window export writes —
export-faithfulness is preserved through the change.

kit_multiples_check() makes the guarantee explicit per forge: after
export, each file's duration-in-bars at the shared grid BPM is compared
to the nearest integer multiple; >0.5% deviation warns with numbers.
export_take_report() bundles export + check; the server /api/export now
returns the multiples block.

Spot-check on real stems (old per-edge vs new length-preserving snap,
top-5 finder candidates re-graded on the exact export window):
  superfreak drums: dev +0.4…+3.6ms → ≤0.01ms; grades C→S, A→A, S→A, S→S
  superfreak bass:  dev −1.1…+3.7ms → ≤0.01ms; grades A→S, B→C, S→B, A→A
  hammer drums:     dev −1.8…+1.9ms → ≤0.01ms; all S stay S
  hammer bass:      dev −2.9…+6.9ms → ≤0.01ms; C→B, A→S, D→C, B→C
Net: duration guarantee achieved; grade moves are the rigid-length
window exposing true seams (some old windows only looked seamless
because per-edge snap trimmed the tail). No systematic regression;
existing superfreak/hammer kit exports untouched (validated, in use).

Tests: +4 (snap length invariance, bar_len_samples, end-to-end exported-
duration-is-exact-bar-multiple on a synthetic stem, multiples-check
flags off-grid). 81 passed.