api: round-trip omits fields whose value is 0 (clef-octave-change, transpose diatonic, stem @default-y)

[webern]

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

What

The api drops elements/attributes whose value is 0, suggesting the writer treats 0 as "unset":

• <clef-octave-change>0</clef-octave-change> dropped — ksuite/k006a_Clefs_sign_G_ln_2_oct_0_Treble.xml (expected has it; actual does not)
• transpose <diatonic>0</diatonic> dropped — custom/transposition.musicxml (the 0 entry is missing; distance 1)
• <stem default-y="0"> loses default-y — ksuite/k002a_Fermatas.xml (distance 1, sole blocker)

Impact (reorder-free candidate files; 550 total)

• drop:clef-octave-change 14, attr:stem@default-y 3, drop:diatonic 3
• The diatonic and stem cases are distance-1 instant wins.

Repro

make dump-api-roundtrip && make classify-api-roundtrip
# build/api/classified.json: signatures drop:clef-octave-change / drop:diatonic / attr:stem@default-y

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