Fix initial clef changing when replacing instrument after an instrument change (#32340)

[pyonpyoco]

Resolves: #32340

This PR fixes #32340 where replacing an instrument via Staff/Part Properties after an instrument change unexpectedly changes the initial clef (staff <defaultClef>).

Use the staff’s current default clef instead, so the dialog does not update the staff <defaultClef> in the instrument-change scenario.

Testing

1. after instrument change: replace instrument → initial clef unchanged (fixed)
2. before instrument change: replace instrument → initial clef changes (expected)
3. another staff without instrument change: replace instrument → initial clef changes (expected)

2026-02-26.0200.32340light.mp4

If you already have a fix in progress, feel free to close this PR.

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