Fix potential excerpt rename not persisting (implements lightweight excerpts)

[manolo]

Resolves #31656
Supersedes #31658

Summary

Part name changes in the Parts dialog now persist after save/reload, even for parts that were never opened as tabs.

This implements the "lightweight excerpts" approach suggested in the review of #31658: instead of fully initializing excerpts just to save their names, we store minimal metadata (~400 bytes) without creating a full Score (several KB per excerpt) saving resources.

Changes

Engraving layer:

• Add MasterScore::addLightweightExcerpt() to add excerpts without initialization
• Add lightweight excerpt write support in MscSaver (name, parts, tracksMapping, initialPartId)
• Add lightweight excerpt read support in MscLoader with <lightweight> marker detection
• Add null checks in initParts(), rebuildExcerptsMidiMapping(), and onWriteExcerpts302() for excerpts without excerptScore
• Add Excerpt::setInited() public setter for deinit support

Notation layer:

• Add MasterNotation::promotePotentialExcerptToLightweight() to promote renamed potential excerpts
• Skip lightweight excerpts when applying styles to all parts
• Skip lightweight excerpts when collecting parts for operations
• Skip writing viewsettings/audiosettings for lightweight excerpts
• Add ExcerptNotation::deinit() and MasterNotation::deinitExcerpts() to restore lightweight state after export

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• Each commit's message describes its purpose and effects, and references the issue it resolves
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• The code compiles and runs on my machine, preferably after each commit individually
• I created a unit test or vtest to verify the changes I made (if applicable)

[manolo]

@cbjeukendrup let me know if the implementation fits your suggestions. If so, I can continue with the two issues you mentioned regarding reset and delete functionality, probably in a separate PR once this is accepted.