Function write_step_fixture 

pub fn write_step_fixture(
    config: &FixtureWriteConfig,
    file_path: &Path,
    test: &str,
    step_index: usize,
    fixture: &Fixture,
) -> Result<()>

Persist a fixture for a single step.

Returns Ok(()) on success, or the first filesystem error encountered. The runner treats write failures as soft errors (logged to stderr, run continues) because a missing fixture never blocks a pass/fail result.