MEASURED_GAIN_TOLERANCE

How far a level measured out of a written file may sit from the gain the plan folded.

One figure for all four backends, so a lowering that is quietly wrong on one of them cannot be accommodated by loosening only that backend's own test. It is wide enough to absorb a lossy codec and an encoder's own dither, and no wider: a fade read at half its range lands inside this, and a fade that ran the wrong way or the wrong length does not.