Two defects surfaced while shipping the hammer (U Can't Touch This) vox
kit extension — both fixed in the shared cut tail, now factored as
_finish_clip():
1. DC offset. Demucs vocal stems carry a small DC bias (measured
−9.6e-5 stem-wide on the hammer catch); every cut inherits it and
longer phrases tripped the grader's B2 dc-offset flag (|mean|>1e-4,
'break it down' cut measured −1.1e-4). Subtlety: a naive
mean-subtract BEFORE fading is undone by the fades — the edge ramps
remove signal asymmetrically and re-introduce up to ~1e-3 DC
(caught by the new regression test, not by the real-data smoke
run). Fix: subtract the FADE-WEIGHTED mean c = Σ(w·x)/Σw (w = fade
envelope), then fade — post-fade mean is exactly 0 per channel AND
edges are exactly 0. On-disk verify of the shipped hammer vox files
reads dc = −0.0e+00, edge = 0.0e+00 across all six.
2. Loop-length drift. cut_phrase_loop placed the tail at exactly
loop_beats × beat_s … then zc-snapped it (±12 ms), re-introducing
the very grid deviation the length-preserving export rule (0759def5)
forbids. The end snap is unnecessary — the fades already guarantee
clickless edges — so the tail is now length-rigid: shipped
08_vox_stophammertime_loop is 4.00 beats to 0.02 ms at BPM 132.51.
Tests: tightened test_cut_phrase_loop_is_exact_beat_multiple from
0.06-beat slack to <1 ms, added test_cut_removes_source_dc_offset
(biased source ⇒ DC-free, unflagged cut). 82 passed.
