api: round-trip drops credit-words justify attribute

[webern]

Split out of #230, which grouped unrelated attribute drops on different elements. One repro per issue.

What

<credit-words> loses its justify attribute on round-trip. Verified on lysuite/ly52a_PageLayout.xml.

Impact

attr:credit-words@justify — 3 reorder-free candidate files.

Repro

make dump-api-roundtrip && make classify-api-roundtrip   # signature attr:credit-words@justify

References

• Split from api: round-trip drops modeled attributes (clef @number, repeat @times, key cancel @location, credit-words @justify) #230 · 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