Excerpt layout temporarily changes linked TAB bend fret values

Issue type

Engraving / linked score state

Description and native reproduction

Laying out an excerpt temporarily changes the linked master score's TAB bend-continuation frets, even though no musical edit is requested. This is confirmed in native C++; a visible desktop reproduction is not confirmed.

Use vibrato-2.mscz (ZIP), the existing official vtest/scores/vibrato-2.mscz fixture:

1. Read the score and lay out the master.
2. Create an ordinary Guitar excerpt with Excerpt::createExcerpt.
3. Locate the first-measure TAB bend chain: pitch 59 → grace-note pitch 60 → main-note pitch 62 (track 4).
4. Lay out the master, then call part->doLayout().
5. Inspect the master note frets before laying out the master again.

Expected: the bend-chain notes keep the starting fret 0 on string index 1. A bend continuation's sounding pitch intentionally differs from its unbent fret pitch.

Actual: the master continuation frets become 1 and 3 rather than 0. The native assertion on the final pitch-62 note reports:

Expected equality of these values:
  chain[i]->fret()
    Which is: 3
  semitones
    Which is: 0

Scope and desktop observation

Returning to the master in the desktop application appears to relayout it and restore the displayed frets. Ordinary Parts → Open Guitar → return to master did not visibly show the mutation in the desktop check. No claim of persistent file corruption, a save/reopen regression, or user-visible undo damage is made here. This report concerns actual linked note properties between native layout calls; the practical UI impact still needs assessment.

Version and validation

Official main 48dcc3d469d7b61d8fb371ba84edba0bc6ba1c06, Ubuntu 24.04.2, Qt 6.10.2, GCC 14, Debug/AddressSanitizer. First affected release is not established.

The native Engraving_TabTransposeTests.bendFretsSurviveExcerptLayoutAndTranspose regression fails without the candidate production change. With it, all 73 Parts and TAB-transposition tests pass. The added test covers both linked scores, the real bend chain's +2-semitone transposition (frets become 2), undo, redo, and repeated layout.

Root cause and prior work

StringData::sortChordNotes treats a bend continuation's deliberate pitch/fret difference as invalid fretting. Subsequent fretting updates propagate to linked notes. The candidate exempts only this mismatch check for notes with an incoming bend, retaining invalid-string/fret handling and allowing the bend's starting note to refret during transposition.

Related history: #29075 was addressed by #29465 with a broad bend-note skip. Commit 671ee95072c7d0c6c877103fd153cdcd74180a3d in #31292 later removed that skip to fix transposition. The narrower candidate is tested against actual bend-chain transposition; it does not restore the old blanket skip. This is a newly isolated excerpt-layout trigger in that known problem area, not a claim of first discovery. Maintainer guidance on whether it belongs with the earlier report is welcome.

Candidate fix: #34856.