[webern]

Summary

<staff-details> kept <staff-lines> but dropped every <staff-tuning>, so a guitar tab came
back with its lines and no record of what the strings are tuned to.

StaffData gains staffTunings, a list of the new StaffTuningData (line, step, alter, cents,
octave). Like staffLines, it is set on the measure whose attributes carry the staff-details and
is not carried forward to later measures. The reader honours the number attribute on
<staff-details>, and reads a <staff-details> that holds only tunings. The writer emits the
tunings between <staff-lines> and <staff-size>, as the schema orders them.

The tuning-step / tuning-alter / tuning-octave translation is shared with the existing scordatura
reader and writer through a small header-only helper (TuningFunctions.h), with no change to
scordatura behavior or diagnostics. A separate StaffTuningData type rather than reusing
AccordData: a staff tuning has a required line, an accord an optional string number.

musuite/testTablature1.xml, testTablature2.xml and testTablature3.xml now round-trip and are
pinned in the baseline. lysuite/ly71e_TabStaves.xml and recsuite/BrookeWestSample.xml lose
their tuning differences but still fail on other staff-details content (staff-type, capo,
show-frets, line-detail), which is not modeled here.

Testing

• New MeasureDataTest cases: tuning round trip, alter and cents, tunings without staff-lines,
  two staves; new MeasureWriterTest case for the written <staff-tuning>
• Existing scordatura tests pass unchanged
• make api-test passes (6195 assertions in 716 test cases)
• make api-roundtrip passes, 445 of 445 pinned files
• make api-roundtrip-discover: 445 pass, no file that passed before now fails
• Unity build (CMAKE_UNITY_BUILD_BATCH_SIZE=0) of the mx target compiles
• make fmt-check passes

References

• Stacked on feat: key signature print-object in mx::api #468