api: round-trip drops key cancel location attribute

[webern]

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

What

The key <cancel> element loses its location attribute on round-trip. Verified on
synthetic/cancel.location.3.0.xml.

Impact

attr:cancel@location — 1 reorder-free candidate file.

Repro

make dump-api-roundtrip && make classify-api-roundtrip   # signature attr:cancel@location

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