api: round-trip drops stem @default-y when value is 0

[webern]

Split from #229.

What

<stem default-y="0"> loses the default-y attribute on round-trip. This is a similar
zero-as-unset bug but has a different root cause from the clef-octave-change and transpose
diatonic cases — it involves the position data attribute handling rather than a simple guard
condition.

Impact

3 corpus files have this as a divergence signature (attr:stem@default-y). Distance-1 sole
blocker for ksuite/k002a_Fermatas.xml.

Files

• Position data attribute emission path (likely PositionFunctions.cpp or similar)
• Needs investigation to identify exact site