Fix/autosave background write

[CubikingChill]

Resolves: #31596 (Maybe)

Move autosave file I/O to background thread to reduce UI stutter

Problem

Autosave runs entirely on the main thread via a QTimer. The file I/O steps — writeFile, copy, and remove — are blocking operations that stutter the UI mid-editing, which is disruptive to workflow.

Solution

After serialization completes on the main thread (unchanged), move the file I/O steps to a background thread using Concurrent::run when MUSE_THREADS_SUPPORT is available.

The serialization itself stays on the main thread because:

• It's read-only on the score data
• It writes to an in-memory Buffer, not disk
• The buffer is fully complete before the background thread starts, so no locking is needed

When threads are not supported, autosave falls back to the existing synchronous behavior.

What is not changed

• Manual save, SaveAs, SaveCopy — all unchanged
• Serialization logic — unchanged
• Backup generation — not applicable for autosave
• Corruption check — not applicable for autosave

How to test

1. Open a large score with multiple parts
2. Edit the score and wait for autosave to trigger
3. Verify no UI stutter occurs during autosave
4. Verify the autosave file is written correctly and can be recovered

• I signed the CLA
• The title of the PR describes the problem it addresses
• Each commit's message describes its purpose and effects, and references the issue it resolves
• If changes are extensive, there is a sequence of easily reviewable commits
• The code in the PR follows the coding rules
• There are no unnecessary changes
• 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)

[igorkorsukov]

The idea is very interesting.

But it's confusing that we have duplicate logic for both MUSE_THREADS_SUPPORT and non-MUSE_THREADS_SUPPORT, and it's currently written in a way that could be different.

How can we do this better?

[CubikingChill]

Changed. Let me know your opinion please.