Improve performance metrics and enhance backup error handling

- Update benchmark results for `full-scan` and `dirty-scan` in `bench/dirty-vs-full.md` to reflect improved performance.
- Refactor error handling in `libmarlin/src/backup.rs` to provide clearer messages when the live database path is missing or invalid.
- Clean up code in `libmarlin/src/backup.rs` for better readability and maintainability.
- Minor adjustments in documentation and test files for consistency.

cli-bin/tests/test.md

@@ -69,6 +69,4 @@ or
 
 ```bash
 ./run_all_tests.sh
-```
-
-Stick that in a shell alias (`alias marlin-ci='…'`) and you’ve got a 5-second upgrade-and-verify loop.
+```