Fix corruption on section break in parts

[mike-spa]

Resolves: #24970

For every other system element, we conventionally set the track to zero. I don't know why it was chosen to set it to muse::nidx (i.e. an invalid value) for LayoutBreak elements. This was causing the section breaks to be saved with spurious staff index information, which was causing them to not be relinked on reading because their staves wouldn't match. Simply setting their tracks to zero seems to resolve both the corruption examples reported in the issue.

The second commit cleans up an unrelated thing about the drawing order (i.e. a cleaner way of making sure that LayoutBreak elements are drawn behind all notation).

[Jojo-Schmitz]

Wouldn't you need to fix

MuseScore/src/engraving/tests/barline_tests.cpp

Line 242 in bbabb14

lb->setTrack(muse::nidx); // system-level element

and

MuseScore/src/importexport/capella/internal/capella.cpp

Line 1373 in bbabb14

lb->setTrack(muse::nidx); // this are system elements

and

MuseScore/src/importexport/mei/internal/meiimporter.cpp

Line 3319 in 3c06d2b

layoutBreak->setTrack(muse::nidx); // this are system elements

too?