wire() registered every mask-panel callback in one 778-line function
behind section comments. Each of the seven fenced sections (computing
the region map, refining a subject's mask, dragging a gradient,
selecting on the photograph, the stack, the edge treatment, adding
layers) becomes its own function, called from wire() in the original
order with the section's own comment kept as its doc comment. The
"adding layers" section was itself over 300 lines and had no further
section markers inside it, so it is split at its own natural seam
between painting/viewing a mask (wire_layers_paint) and working the
parts and add-mask buttons (wire_layers_parts); the second half gets an
introductory doc line since there was no comment of its own to reuse.
Locals declared just for one section's closures (running, refining,
dragging) move into that section's function instead of staying in
wire().