Polars 2.0: eight silent breaking changes in upgrade guide

Polars 2.0 makes the streaming engine the default for LazyFrame.collect(), and eight changes in the official upgrade guide can alter pipeline output without raising errors. No release date is given, and as of 8 October 2026 sources disagree whether 2.0.0 final has shipped.

• LazyFrame.collect() now defaults to the streaming engine instead of in-memory
• Row order is no longer guaranteed for unpivot, group_by and joins
• read_csv now routes through scan_csv().collect(), with dtypes renamed to schema_overrides
• Selector operators &, | and ^ are now element-wise, not set operations