Refactor partition iteration to support a generic `on_partition` callback executed after each parallel partition completes. Split the logic into bounded and unbounded paths; the bounded path uses an `AtomicUsize` to enforce row limits, while the unbounded path eliminates atomic contention to improve throughput. Additionally, integrate a progress bar into the dump command by passing an increment callback to `idx.dump()`, ensuring proper initialization and cleanup.
Eric Coissac