Split unvalidated metadata out

[philpax]

OWL Control can occasionally fail to overwrite its own metadata during validation:

2026-01-16T10:53:54.960841Z  INFO owl_control::record::local_recording: Validating recording at D:\Game Rec for Owl\1768560685
2026-01-16T10:53:55.046421Z ERROR owl_control::record::local_recording: Error validating recording on stop: Validation failures: Error renaming metadata temp file: Os { code: 32, kind: Uncategorized, message: "The process cannot access the file because it is being used by another process." }
2026-01-16T10:53:55.061033Z  INFO owl_control::record::recorder: Recording stopped

As a follow-up fix to @Clydingus's rename workaround, this uses a different filename for the initial metadata, so that we are less likely to encounter a write-lock of any kind when writing the validated file out. This will also check for existing metadata.json files to ensure that previous versions can still be uploaded.

I'm not merging this in yet as I haven't fully tested it yet, but it should be pretty easy to land in a patch release.