api: round-trip drops part-group

[webern]

Split out of #219 (investigate api dropping elements marked support=full), one element per issue.

What

<part-group> is dropped on api round-trip, affecting ~373 corpus files. data/api.features.xml audits it as full.

To be determined (the point of this issue):

1. a genuine impl round-trip bug - we model part-group but drop it on write (impl fix), or
2. api.features.xml overstating support - it is really partial/none and the audit is wrong (audit correction).

Repro

make dump-api-roundtrip && make classify-api-roundtrip
# build/api/classified.json: <part-group> among the drop signatures

References

• Split from investigate api dropping elements marked support=full on round-trip #219 · Part of api: expand corpus coverage of the api round-trip pass-list #208