Expose FretDiagram setDot/setMarker/setBarre to plugin API

[dheerajchand]

Summary

Add a FretDiagram wrapper class to the QML plugin API that exposes the methods needed to programmatically set dots, markers, and barres on fretboard diagrams from plugins.

Currently, plugins can create a FretDiagram element and set its grid properties (fretStrings, fretFrets, fretOffset), but cannot set individual dot positions, open/muted markers, or barres. This makes it impossible to insert complete fretboard diagrams from a plugin — only empty grids.

Changes

src/engraving/api/v1/elements.h:

• Add #include "engraving/dom/fret.h"
• Add FretDiagram wrapper class inheriting from EngravingItem
• Expose Q_INVOKABLE methods: setDot(), setMarker(), setBarre(), clear()
• Expose Q_PROPERTY accessors: strings, frets, fretOffset, showNut

src/engraving/api/v1/elements.cpp:

• Add API_WRAP(FretDiagram) to the wrap() function

Plugin usage after this change

var fd = newElement(Element.FRET_DIAGRAM)
fd.strings = 6
fd.frets = 4
fd.fretOffset = 7  // fret 8

// Set dots (0-based string index, 0 = highest string)
fd.setDot(5, 1)    // root on low E
fd.setDot(3, 1)    // b7 on D
fd.setDot(2, 2)    // 3 on G

// Set markers
fd.setMarker(4, 2) // mute A string (2 = cross)
fd.setMarker(1, 2) // mute B string
fd.setMarker(0, 2) // mute high e

cursor.add(fd)

Context

This resolves #32798. The FretDiagram dom class already has full setDot(), setMarker(), and setBarre() implementations — they just aren't exposed to the QML plugin API. This PR follows the exact same wrapper pattern used by Note, Chord, Lyrics, and other existing API wrapper classes.

The Chord Library plugin is a concrete use case — it provides a searchable jazz guitar voicing library but currently can only insert empty grids because of this limitation.