Bug-fix release. Results are unchanged from 1.2.0 for a board that
solves cleanly; the fixes are in the in-KiCad overlay push, pad copper
selection and error reporting.

Note for anyone coming from 1.1.0 or earlier: 1.2.0 changed the physics
model (exact SMD and THT pad copper, populated THT holes conducting as
their solder plug and lead, slotted holes as true stadiums) and fixed an
adaptive barrel-refinement bug that could make via-field results read up
to ~13% low. Numbers for an unchanged board differ from 1.1.0 - re-run
any board you track across versions.

Fixed:
- Overlay push: a locked reference image silently survived removal and a
  new one was stacked on top of it. KiCad reports the failure per item
  while the overall request still reads OK; it is now checked, and the
  layer is reported and skipped instead.
- Overlay push: a run covering fewer layers than the previous one left
  the earlier solve's heatmap on the unused slots, where it read as
  current. Those slots are now cleared.
- Overlay push: the whole push is one commit, so a single undo reverts
  it rather than just the last layer.
- Through-hole pad copper was always read from F.Cu even when the joint
  protrudes on B.Cu, mis-sizing the modelled solder coat for pads sized
  differently per copper layer. The solder side is now probed first.
- A failure before the output directory existed - a broken plugin
  Python environment, typically - reported nothing at all on screen.
  The error figure now falls back to the temp directory.
- Pads sitting on no single copper layer are noted rather than silently
  skipped, and the frequency field keeps its specific rejection reason
  ("1,500" is a thousands separator, "-5" is negative) as the other
  numeric fields already did.

The KiCad overlay push remains experimental and opt-in (off by default).
It writes reference images to User.9-User.12 and replaces what is on
those layers.