feat: clef additional and after-barline in mx::api

[rpatters1]

Human Summary

Add Bool ClefData::additional and map MusicXML's clef@additional in both directions. unspecified (the default) omits the attribute; yes/no are
written verbatim.

Summary

Also fix a write-side gap found while adding it: ClefData::location was
already populated by MeasureReader from clef@after-barline, but
PropertiesWriter never emitted the attribute, so the value was silently
lost on write. afterBarline and beforeBarline now write after-barline
yes/no; unspecified and midMeasure emit nothing, since MusicXML ignores the attribute on mid-measure clefs. Document ClefLocation, which had no comment.

Tests cover yes/no/omitted for both attributes, plus an all-fields
round-trip over ClefData that pins the remaining eight fields. That test
records one deliberate normalization: writeStaffNumber=no on a
single-staff part reads back as unspecified, because the knob records
only a divergence from the automatic rule and the automatic rule already
omits the number there. The emitted XML is identical either way.

data/synthetic/clef.3.0.xml and clef.3.1.xml still fail the api
roundtrip on unmodeled position, font, color, and id attributes, so
neither is pinned. The discovery pass-list is unchanged at 297.

[webern]

This one is pretty straightforward and looks good. Easy to approve from the phone. 👍