Preserve existing profiles when a save is interrupted - #129

SaveProfileNew currently opens the destination with overwrite enabled. If writing fails after the file is opened, a valid profile can be replaced with incomplete XML.

Write the existing serialized XML to a unique temporary file beside the destination, flush it, then replace the existing profile or move the completed file into place for a new profile. Remove temporary files after failures. The profile schema, serializer and UTF-8 encoding stay the same.

Regression tests cover creation, replacement, interrupted writes with and without an existing destination, locked and read-only destinations, temporary-file cleanup, and the real BackingStore.SaveProfileNew path.

Validation on Windows, .NET SDK 8.0.425, Release x64:

• All 6 cases in ProfilePersistenceTests passed as part of the combined validation run.
• 136 related tests passed with both proposed fixes present; this includes existing profile serialization, migration, load preparation and output ownership tests.
• The patch applies independently to upstream commit 36502408cbba17fd20d84e6b258f2258714a39c6.

This change preserves the existing save exception/return contract. Editor error reporting and rename behavior need a separate fix. Tests use disposable local files; process termination, power loss and network filesystems were not tested.

Focused check:

dotnet test DS4WindowsTests/DS4WindowsTests.csproj -c Release -p:Platform=x64 --filter FullyQualifiedName~ProfilePersistenceTests