fix(musicxml): correct barre fret export for shifted chord diagrams

[skoliveira]

Barre notes written via the 'unwritten barres' fallback path in writeMusicXml(FretDiagram) were emitting relative fret numbers instead of absolute ones, while every other code path (dots, first-fret tag) correctly applied fretOffset.

This caused any chord diagram with first-fret > 1 to render the barre at the wrong position in importing applications.

Fix: add item->fretOffset() to the fret value in both the bStarts and bEnds loops, consistent with how dot frets are already handled.

Example: a Csus4 barre at position 3 (fretOffset=2) was exporting fret=1 instead of the correct fret=3.

Fixes: barre chords in all shifted chord diagrams (first-fret > 1)

Resolves: #32958

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[coderabbitai]

📝 Walkthrough

Walkthrough

The MusicXML export for FretDiagram objects now adds the diagram's fretOffset() when writing <fret> values for barre-related frame-note elements. In writeMusicXml(const FretDiagram* item, XmlWriter& xml), both loops emitting barre start notes and barre end notes use j + item->fretOffset() instead of j, so exported <fret> entries reflect the diagram's fret offset.

🚥 Pre-merge checks | ✅ 4 | ❌ 1

❌ Failed checks (1 warning)

Check name	Status	Explanation	Resolution
Docstring Coverage	⚠️ Warning	Docstring coverage is 0.00% which is insufficient. The required threshold is 80.00%.	Write docstrings for the functions missing them to satisfy the coverage threshold.

✅ Passed checks (4 passed)

Check name	Status	Explanation
Title check	✅ Passed	The title clearly and concisely describes the main change: correcting barre fret export for shifted chord diagrams, which is the primary objective of the PR.
Linked Issues check	✅ Passed	The code changes directly address the core requirement from issue #32958: applying fretOffset() to barre fret exports to use absolute numbering, matching non-barre note handling.
Out of Scope Changes check	✅ Passed	All changes are directly scoped to fixing the barre fret export issue; no unrelated modifications or out-of-scope changes are present in the changeset.
Description check	✅ Passed	PR description comprehensively covers the bug, its impact, the implemented fix, and references the resolved issue #32958. All required template sections are completed with checkboxes marked appropriately.

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✨ Finishing Touches

🧪 Generate unit tests (beta)

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