Fix corruption in scores with MMRests and repeat courtesies

[miiizen]

Resolves: #31364
In addition to the corruption, the two 6/8 markings in the soprano saxophone part is incorrect. There should only be one before the repeat barline.

We weren't updating MMRest's underlying segments in all situations in ModifyDom::sortMeasureSegments. This lead to stale time signatures being copied to MMRest segments and causing corruptions.

[davidstephengrant]

There are still some problems.

1. Open the test score attached to Issue 31364
2. Add an end repeat barline at the red 6/8 time signature
3. Click on the Alto Saxophone part tab
4. Open Format > Style > Clefs, key & time signatures
5. Repeatedly check and uncheck Clefs, key & time signatures at repeat and jumps > At repeats, only show changes before the barline
   RESULT: Multiple unwanted 6/8 time signatures start appearing
6. Ensure that At repeats, only show changes before the barline is unchecked, then OK from Style dialog
7. Delete the notes in the bar following the repeat barline (so the last two bars become a multirest)
   RESULT: The expected number of time signatures are drawn (one parenthesized time signature before the barline, and one without parentheses after)
8. Open Format > Style > Clefs, key & time signatures
9. Check Clefs, key & time signatures at repeat and jumps > At repeats, only show changes before the barline
   EXPECTED: A single time signature is drawn before the repeat barline
   ACTUAL: Time signatures both before and after the repeat barline

[davidstephengrant]

Screencast.From.2026-02-06.12-27-33.mp4

[miiizen]

@davidstephengrant This should be resolved now!