Polars 2.0 RC makes streaming the default LazyFrame engine

The Polars 2.0 release candidate makes the streaming engine the default when collecting LazyFrame queries, with Polars estimating aggregate streaming performance could be 5x faster.

The change can alter observable row order for operations including joins, group-bys, and unpivots, though users can set maintain_order=True when needed.

The release also introduces stricter behavior for lossy type coercion, dataframe concatenation, and ambiguous casts, alongside typed errors for removed attributes and arguments.