Plugin API: Score.appendPart() does not apply tablature staff type from instrument template

Summary

Score::appendPart(const InstrumentTemplate*) — the function backing the plugin API's Score.appendPart(id) and Score.appendPartByMusicXmlId(id) — does not apply the template's staffTypePreset. As a result, calling appendPart("guitar-nylon-tablature") from a plugin produces a standard 5-line staff instead of a tablature staff. Same issue affects every instrument whose template carries a non-default StaffType preset (all TAB variants, etc.).

Repro (from any plugin)

curScore.startCmd()
curScore.appendPart("guitar-nylon-tablature")
curScore.endCmd()

Expected: a TAB staff (6 lines, tab clef, fret-number rendering).
Actual: a standard 5-line staff with treble clef.

Root cause

The function lives at src/engraving/dom/score.cpp:4371. In current HEAD it reads:

Staff* staff = Factory::createStaff(part);
StaffType* stt = staff->staffType(Fraction(0, 1));
staff->init(t, stt, int(i));

Factory::createStaff runs Staff::Staff(Part*), which calls initFromStaffType(0) — assigning the default STANDARD preset. So staff->staffType(0) returns a pointer to that default standard StaffType.

That pointer is then handed to Staff::init as its second argument:

// staff.cpp:1372
const StaffType* pst = staffType ? staffType : t->staffTypePreset;

Because staffType is non-null, t->staffTypePreset is never consulted — the staff stays standard regardless of what the instrument template requested.

This is a follow-up bug to #29478 ("Set default clefs when creating parts", commit 451b451bd8). That PR correctly replaced the old hand-rolled body of appendPart with a delegation to Staff::init, but the vestigial stt local left over from the previous body got threaded into the new init call.

Scope

• Score::appendPart(const InstrumentTemplate*) is only invoked by the plugin API (src/engraving/api/v1/score.cpp:139, 155). The Instruments-panel "Add instrument" flow uses a different path (Excerpt::cloneStaff) and is unaffected.
• Bug has been latent in some form since 2012 (commit dc5fda47f3, Werner Schweer) — the original hand-rolled body never applied the preset either. Plugin authors have been working around it by asking users to add the tablature instrument manually via the Instruments panel.

Proposed fix

One-line change: pass nullptr instead of stt, letting Staff::init fall through to t->staffTypePreset. Happy to send a PR with the fix and regression tests if maintainers agree on the approach.

Read src/engraving/dom/score.cpp:4371 and the Staff::init logic in staff.cpp:1372, then trace the plugin entry points in src/engraving/api/v1/score.cpp. Verify appendPart for guitar-nylon-tablature and other non-default staff presets produces the template's staff type, with regression coverage for the plugin API behavior.