The reverb tail shipped with `apad=pad_dur=2.5` on the reasoning that ffmpeg
would otherwise chop the echo at the last sample. That reasoning was wrong, and
the verify stage is what exposed it: REVOLUTION came out 207.64s against a
predicted 207.40, and chasing the 0.24s led to the actual behaviour.

Measured, four ways:

    no apad   -> 5.240s wet, 0.000s trailing silence   (full ring, nothing cut)
    apad=0.05 -> 5.290s wet, 0.050s trailing silence
    apad=0.35 -> 5.590s wet, 0.350s trailing silence
    apad=2.5  -> 7.740s wet, 2.500s trailing silence

aecho extends its OWN output by its longest tap (240ms for subtle), so the ring
was never at risk. Every second of apad simply became trailing digital silence,
one for one. The rendered record therefore ended with ~2.3s of pure zeros —
which is precisely the defect PLN flagged at the other end of the record
("trim leading silence tho"), reintroduced by me at the close.

Two things worth keeping from how this surfaced. The 0.24s mismatch was real
information, and the tempting fix — `atrim` the output back to the predicted
length — would have clipped the end of the ring while making the check go green:
the prediction was wrong, not the audio. And the fix only became findable
because verify compares against a number derived from intent rather than from
whatever the renderer happened to produce.

expected_dur is now duration + aecho's longest tap, exactly. REVOLUTION: 205.14s.