refactor: apply clippy::get_first

[shulaoda]

Summary

• In src/overflow.rs, replaces lists.get(0)? with lists.first()? in last_item_shape, and args.get(0)? with args.first()? in maybe_get_args_offset.
• Addresses clippy::get_first, run as a single-rule pass: cargo clippy -- -A clippy::all -W clippy::get_first.

slice::first() and slice::get(0) return identical Option<&T> values; preferring first() makes the intent explicit.

[ytmimi]

Going to close until we decide on which clippy lints we want to enforce in the codebase.