Refactor port scanning and improve batch size handling

[0xxreacher]

Fix #1 - Wrong batch size in warning message (opts.batch_size → batch_size): The warning was showing the raw CLI value, not the actual adjusted value that ran. Users would try lowering a number that wasn't what the scanner used.

Fix #2 - or_insert_with(Vec::new) -> or_default(): Idiomatic Rust, clippy::pedantic flags the old form. Functionally identical but cleaner.

Fix #3 - scripts_to_run.clone() moved out of loop: Was cloning the entire Vec once per IP. Now iterates by reference (for script_f in &scripts_to_run) and only clones the individual ScriptFile when mutation is actually needed. Zero behavior change, much better perf at scale.

Fix #4 - Removed redundant debug!("Call format {call_f}"): The output! macro right above it already logs the full call format string. Duplicate log at a different level adds noise.

Fix #5 - usize::MAX fallback → DEFAULT_FILE_DESCRIPTORS_LIMIT: The most dangerous one. If try_into() ever failed, the returned usize::MAX would make infer_batch_size think the system has infinite file descriptors and skip all safety adjustments potentially crashing with a massive batch size. Now falls back to the same conservative 8000 limit the rest of the code uses.

[bee-san]

Thanks @0xxreacher! Your batch-size warning fix already landed through #921, where @DanHouseman credited you (you're also co-author on that commit). I merged master in and re-applied the rest of your cleanups on top:

• scripts_to_run is iterated by reference and only the script being run is cloned. Before, the whole list was cloned for every IP.
• or_default() instead of or_insert_with(Vec::new).
• The usize::MAX fallback in adjust_ulimit_size is now DEFAULT_FILE_DESCRIPTORS_LIMIT. Good catch: usize::MAX would have skipped every batch-size adjustment.
• The 'aveage' typo is fixed.

I left out the FIX #n comments and the constant-only test, and I kept the Call format debug log because it's at a different log level from the output! line. CI is green on all four platforms.