api: clef number attribute does not round-trip

[webern]

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

What

The clef number attribute does not round-trip. Investigation (PR #267) found the divergence is the
reverse of a plain drop: for single-staff parts mx added a spurious number="1" on mid-measure
clef changes, where the source omits it (1 is implied). The classifier signature attr:clef@number
covers any mismatch in either direction.

Verified on lysuite/ly46c_Midmeasure_Clef.xml.

Impact

attr:clef@number — 29 reorder-free candidate files (the largest single attribute signature in the worklist).

Repro

make dump-api-roundtrip && make classify-api-roundtrip   # signature attr:clef@number

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
• Fixed by PR fix(api): clef number round-trip #267