Fix mock CKAsset temp path in previews

[Rspoon3]

Summary

• Write CKAsset files in MockCloudDatabase using the data manager temporary directory instead of root.
• Add a test to assert assets are stored under the temporary directory.

Why

Preview/test CloudKit sharing with BLOB columns fails because assets are written to / (read-only).

Error

"🟣 Scorekeeper/GameFeature.swift:90: Caught error: Error Domain=NSCocoaErrorDomain Code=642 "You can't save the file "e8525bd6-4718-4db9-827b-67f72459f877" because the volume "Macintosh HD - Data" is read only." UserInfo={NSFilePath=/e8525bd6-4718-4db9-827b-67f72459f877, NSURL=e8525bd6-4718-4db9-827b-67f72459f877 -- file:///, NSUnderlyingError=0x60000111bcc0 {Error Domain=NSPOSIXErrorDomain Code=30 "Read-only file system"}}"

Steps to Reproduce

1. In Scorekeeper/Schema.swift, add a Data column to Game:

   @Table struct Game: Identifiable {
     let id: UUID
     var title = ""
     var attachment: Data = Data()
   }

2. Update migration: "attachment" BLOB NOT NULL DEFAULT x''
3. Seed with a non-empty value: Game.Draft(id: UUID(1), title: "Family gin rummy", attachment: Data([0x01]))
4. Build/run preview and trigger sharing (syncEngine.share(record:)).

Expected Result

Sharing succeeds (or the mock sharing view appears in preview) without filesystem errors.

Actual Result

Sharing fails during syncEngine.share(record:) with NSCocoaErrorDomain 642, and the file path is rooted at /.

Notes

• Does not occur before adding the Data column.
• CKAsset staging uses a temp directory that resolves to / in preview/test contexts.
• Suggests preview/test should use a writable temp directory (e.g. NSTemporaryDirectory) or allow a dependency override for asset temp directory.

Testing

• Added MockCloudDatabaseTests.assetsUseTemporaryDirectory

[mbrandonw]

Hey @Rspoon3, thanks for this! We made a few changes (using InMemoryDataManager in previews and updated the test), but will merge and release soon.