Exported part scores still contain links to master score

Issue type

Import/export issue

Description with steps to reproduce

If you export a score's parts using the command line, the generated mscz files still contain linkedTo elements referring to the original master score, which no longer exists alongside the part.

When the newly-generated standalone part(s) is loaded into MSS, there are a lot of EID errors (~ one per engraving item?), which are apparently harmless since the load ignores them and continues successfully, but might be worth fixing:

17:22:03.449 | ERROR | main_thread     | TRead::readItemLink | ASSERT FAILED:    eid.isValid()    /Users/ajuncosa/src/MuseScore/release/src/engraving/rw/read460/tread.cpp:592
17:22:03.449 | ERROR | main_thread     | EIDRegister::itemFromEID | ASSERT FAILED:    iter != m_eidToItem.end()    /Users/ajuncosa/src/MuseScore/release/src/engraving/infrastructure/eidregister.cpp:84
17:22:03.449 | ERROR | main_thread     | TRead::readItemLink | ASSERT FAILED:    mainElement    /Users/ajuncosa/src/MuseScore/release/src/engraving/rw/read460/tread.cpp:595
17:22:03.449 | ERROR | main_thread     | TRead::readItemLink | Link failed: Main linked element not found for Staff at 0/1
...

Steps:

1. Export a score's parts using the command line, like: mscore --score-parts score.mscz -o score-parts.json
2. List the parts using jq: jq <score-parts.json '.parts'
3. Get an mscz of the first part: jq < score-parts.json -r '.partsBin[0]' | base64 --decode > part0.mscz
4. Open the part0.mscz file in MuseScore Studio

Actual: a bunch of EID errors are printed to the terminal (if in Debug build, the application stops at the first assert).
Expected: the score is loaded cleanly

Supporting files, videos and screenshots

In which versions of MuseScore Studio is this issue present?

4.6.5 - 4.7.5

Regression

Yes, this used to work in a previous version of MuseScore 4.x

Operating system

macOS 26.5

Additional context

No response

Checklist

• This report follows the guidelines for reporting bugs and issues
• I have verified that this issue has not been logged before, by searching the issue tracker for similar issues
• I have attached all requested files and information to this report
• I have attempted to identify the root problem as concisely as possible, and have used minimal reproducible examples where possible

Reproduce the issue with the command-line --score-parts steps and inspect the generated part.mscz in MuseScore Studio. Start with the linkedTo data and the reported read paths in engraving/rw/read460/tread.cpp and engraving/infrastructure/eidregister.cpp. Done means exported standalone parts load cleanly without EID errors.