api: round-trip drops clef-octave-change when value is 0

[webern]

Split from #229.

What

PropertiesWriter.cpp:218 uses if (inClefData.octaveChange != 0) as a guard, which drops
<clef-octave-change>0</clef-octave-change> from the output. Zero is a valid value meaning
"no octave transposition" and must be emitted when present in the source.

Fix

Add bool isOctaveChangeSpecified to ClefData (mirrors the existing isLineSpecified pattern).
Set it in the reader when <clef-octave-change> is present. Check it in the writer instead of
!= 0.

Impact

14 corpus files have this as a divergence signature (drop:clef-octave-change).

Files

• src/include/mx/api/ClefData.h
• src/private/mx/impl/PropertiesWriter.cpp:218
• Reader site that parses <clef-octave-change>