feat: add measure text and measure-numbering staff to mx::api

[rpatters1]

Human Summary

Add fields for <measure text="..."> and <measure-numbering staff="N"> to MeasureData. With the number of measure-number-related fields up the 3 in MeasureData it may make sense to refactor them into dedicated MeasureNumberData class. This would be a breaking change, so I did not include that in this PR. I could rework it that way, though, if it makes sense to do so.

Summary

Two <measure>-scoped attributes were being dropped on import and never written on export. Both
are now exposed on MeasureData and wired symmetrically through mx::impl.

displayedNumber (std::optional<std::string>) carries <measure text="...">: the measure
number as printed, when that differs from the number that identifies the measure. It is fully
independent of MeasureData::number ‚Äî number's existing "empty means index + 1" normalization
does not touch it. An empty string is not emitted, since text="" is not legal MusicXML and
core::MeasureText would repair it to "-"; suppressing the printed number is what implicit
is for.

measureNumberingStaffIndex (std::optional<int>) carries <measure-numbering staff="N">, the
staff used as the reference point for vertical positioning of the number. Zero-based, converted
across the impl boundary, matching the sibling measureNumbering* fields. Absent means the top
staff.

Both fields got MXAPI_EQUALS_MEMBER lines.

No roundtrip-baseline.txt entries added. Discovery shows synthetic/measure.3.1.xml moved from
failing on text to failing on the still-unmodeled <measure id=...>, and
synthetic/measure-numbering.4.0.xml still fails on the unmodeled print-style-align attributes.
Neither pass is available yet, so neither is attributable to this change.

Testing

• Five new MeasureData cases cover round-trip, omission when absent, and empty-string
  suppression: displayedNumberRoundTrip, displayedNumberAbsentOmitsAttribute,
  emptyDisplayedNumberOmitsAttribute, measureNumberingStaffRoundTrip,
  measureNumberingStaffAbsentOmitsAttribute
• make api-test passes (5237 assertions in 468 test cases)
• make api-roundtrip passes (295 of 295 pinned files)
• make fmt-check passes

[webern]

Nice. I agree with a quicker fix now instead of redesigning or refactoring right this moment.

[webern]

Meaningful only when measureNumber != unspecified is not ideal. But there are plenty of patterns already like this and it's easier/faster to just do this than to redesign it now. So I agree with the choice.