feat: mix each loop within its track as well as the track itself

[HenryMBaldwin]

• Add per-loop level and pan to Settings, feeding the track's own.
• Compose pan in position space: a loop nudges its track along the row rather than replacing it.
• Add the loop picker and a full-height slider, reached by the row/column button on the volume and pan screens.
• Step back one screen at a time on the side button, out to the loops.
• Save each loop's trim in its clip entry, defaulting to flat.

Levels multiply, so a loop trim and the same cut on the track fader land in the same place. Pans cannot: cascading two constant-power stages would put 1.414 squared on one channel, so compose_steps adds the loop's offset from centre to the track's step and holds it inside the row.

A pad returns to flat when it empties, so a fresh take does not inherit shaping that is invisible from the loops screen.