feat: add --incremental alias for --iterate

[Frostand]

Summary

• accept visible --incremental as an alias for the existing --iterate flag
• keep a single internal state field and add a parser regression test
• document the alias in the user guide and NEWS.md

Closes #621.

Testing

• cargo test incremental_alias_enables_iterate --all-features — 1 passed
• cargo fmt --all --check
• cargo clippy --all-targets --all-features -- -D warnings
• git diff --check

Tooling disclosure: this change was prepared with OpenAI Codex and validated with the commands above.