api: metronome parentheses attribute is dropped despite TempoData::isParenthetical existing

[webern]

What

<metronome parentheses="yes"> loses its parentheses on an api round-trip. The api model
already has the field — TempoData::isParenthetical (a Bool) — but it dangles: neither
MetronomeReader sets it nor DirectionWriter emits it, so a parenthesized metronome mark
comes back bare.

Verified on lysuite/ly31c_MetronomeMarks.xml:

• expected: <metronome parentheses="yes">
• actual: <metronome>

Impact (classifier signature attr:metronome@parentheses)

23 candidate files, 2 of them distance-1 (lysuite/ly31c_MetronomeMarks.xml,
musuite/testTempo1.xml).

Fix shape

Wire the existing field: reader maps core::Metronome::parentheses() to the Bool, writer
emits it when not unspecified.

References

• Part of api: expand corpus coverage of the api round-trip pass-list #208