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.