Function process_file 

pub fn process_file(index: usize, path: &Path, ops: &BatchOps) -> FileResult

Processes a single file, running all requested operations.

NEVER calls crate::commands::validate / stats / list — those have side effects (process::exit). We call lib3mf-core APIs directly:

• model.validate(level) → ValidationReport
• model.compute_stats(&archiver) → ModelStats
• archiver.list_entries() → Vec<String>