How far the melody was moved, in semitones. Transposing the melody by this
much puts it in key, which is the key the chords are in.
The key the chords are in, whole.
A key/scale alone says nothing about how it is written, so a result
carrying one would report whichever spelling its pitch classes read best
from, and an Ab minor handed in would come back out a G# minor. It matters
most under key: 'infer', where this is the only account of which key was
chosen and a caller has nothing else to spell the chord spans from.
One span per chord change, each starting on a harmonic-rhythm boundary.
Each melody note's role in the chord sounding under it, once chosen.
The chosen transpose, key, chord path, and per-note roles.