Third and last calibration of shape_of() against real audio. perfect.tidal's d4
measured |▇▇▆▆▇▇▆▅▆▆▇▆▃| — visibly steady for twelve bins — and was reported
"DECAYING, 13 dB down, never recovering, almost certainly an xfade tail" on the
strength of the thirteenth. The capture window had simply closed inside a gap
between events.

The midpoint-crossing rule added in 708c2aef cannot catch this: a dip in the last bin
crosses the midpoint exactly ONCE, which is the signature of a genuine fade. So ask a
different question — does the verdict survive dropping the final bin? If the inner
trend is flat, the final bin WAS the verdict, and the answer is STEADY with the edge
dip named explicitly rather than hidden.

A real decay is unaffected, including one that only begins in the last three bins
(still DECAYING). Eight synthetic shapes now separate cleanly: steady-with-edge-dip,
monotonic decay, late decay, gated-riser-dipping-at-the-end, spiky-sparse, died,
steady, rising.

Pattern across all three fixes to this function: every false positive came from
answering a question about SHAPE with a pair of summary statistics. Each fix replaced
a statistic with the structural question that actually distinguishes the cases —
does it recur, does it survive without its last sample. The verdict text now says
which, so the reading can be argued with instead of believed.