Prevent music library loss during reload

[stevenmunn312-tech]

Summary

• block autosave until the active project and its music library finish hydrating
• wait across overlapping music-library restores before capturing a workspace
• skip page-hide saves while restoration is in progress
• add a regression test for overlapping restores

Verification

• npm test -- src/state/tests/musicLibraryPersistence.test.ts src/state/tests/musicStore.test.ts src/state/tests/workspacePersistence.test.ts
• npm run build