test: add regression tests for Live2D review blockers (#350)

* test: add regression tests for review blockers

- backup-clean --json actually deletes old backups
- flatten copies motion Sound assets
- save_model uses atomic write (tempfile + os.replace)
- auto_backup deduplicates on content, not wall-clock time
- save_model cleans up temp files on error

* test: use CliRunner in backup-clean regression test

The previous test deleted files manually, which would pass even if the
CLI regressed. Now it invokes 'backup-clean --keep 2 --json' through
CliRunner and asserts both disk state and JSON output.

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Co-authored-by: zyzly0705 <zyzly0705@users.noreply.github.com>