Enabling the next iteration of the borrow checker on nightly

The Rust team is enabling Polonius Alpha, the next iteration of the borrow checker, on nightly builds ahead of planned stabilization later in the year. Polonius Alpha introduces flow-sensitive borrow checking of lifetime outlives relationships, allowing code that NLL incorrectly rejects — most notably the classic 'get or insert' HashMap pattern. Performance impact across the top 10,000 crates on crates.io is generally minimal, with the worst observed regression being 2-3x in borrow-heavy crates. Users on nightly can opt out via `-Zpolonius=off`. The team is seeking community feedback on performance regressions, unsoundness, and diagnostic issues before full stabilization.