[$] Cross-crate forward declarations in Rust

Rust has several ad-hoc mechanisms to allow user code to influence how thestandard library executes. For example, custom panic handlers and customallocators both involve registering functions that core standard library codeneeds to call. With more similar features planned,Jana Donszelmann spotted the developing pattern and put together ageneral solution, introducing a safer, checked analogue of C's weak symbollinking. At Kangrejos 2026, she explained how this new language feature works,and proposed using it to clean up the kernel's handling of integer overflows inRust code.