[webern]

Summary

MeasureWriter writes each staff starting where the previous staff ended. A direction at the start
of a later staff was written there, before the <backup>, and DirectionWriter added an invented
<offset sound="yes"> (e.g. -32) to reach its real tick. Harmony and a standalone <sound> got
the same offset; a figured-bass, which has no <offset>, simply came out in the wrong order.

When the first pending direction of a staff is behind the write position and not later than the
staff's first note, the writer now backs up to it first, so it follows the <backup> as it does in
the source. Writer-only; no api change.

Deliberately left as before:

• a direction exactly where the previous staff ended (the api data cannot tell which staff's end or
  start it belongs to)
• a direction later than the staff's first note (the note loop places it)
• later leading directions at other ticks, and staves with directions but no notes

Four corpus files now round-trip and are pinned:

• lysuite/ly43e_Multistaff_ClefDynamics.xml
• musetrainer/Canon_in_D_3.xml
• musetrainer/J._S.Bach-_Air_on_the_G_String_Piano_arrangement.xml (also needs fix: write barlines where their location says they belong #470)
• musetrainer/Maple_Leaf_Rag_Scott_Joplin.xml (also needs fix: write barlines where their location says they belong #470)

Across the failing corpus, files with an invented add:offset drop from 34 to 13 and files with
reorder:measure from 49 to 29.

Testing

• New MeasureWriterTest cases: direction, harmony and figured-bass at a later staff's start,
  a single-note staff, a middle barline at the previous staff's end, plus two that pin the cases
  left alone (direction at the previous staff's end, direction after the first note)
• Five of the seven new tests fail without the fix
• make api-test passes (6224 assertions in 730 test cases)
• make api-roundtrip passes, 451 of 451 pinned files
• make api-roundtrip-discover: 451 pass, against 447 on the commit below; no file lost its pass
• make fmt-check passes

References

• Stacked on fix: write barlines where their location says they belong #470