api: round-trip drops modeled attributes (clef @number, repeat @times, key cancel @location, credit-words @justify)

[webern]

Surfaced by the reworked round-trip classifier (PR #225; part of #212).

What

Several attributes are read but not written back:

• <clef number="..."> loses number (mid-measure / multi-staff clefs) — lysuite/ly46c_Midmeasure_Clef.xml
• <repeat times="5"> loses times — lysuite/ly45a_SimpleRepeat.xml (verified: expected times="5", actual none)
• key <cancel location="..."> loses location — synthetic/cancel.location.3.0.xml
• <credit-words justify="..."> loses justify — lysuite/ly52a_PageLayout.xml

Impact (reorder-free candidate files; 550 total)

• attr:clef@number 29, attr:repeat@times 3, attr:cancel@location 1, attr:credit-words@justify 3

Repro

make dump-api-roundtrip && make classify-api-roundtrip
# build/api/classified.json: signatures attr:clef@number / attr:repeat@times / attr:cancel@location / attr:credit-words@justify

References

• Part of api: expand corpus coverage of the api round-trip pass-list #208; feeds api: implement ranked fixes and grow the round-trip corpus baseline #213
• Surfaced by PR audit: classify api round-trip failures by measured divergence and rank a worklist #225