Switching to Rust's own mangling scheme on nightly

Rust's nightly compiler will switch from C++'s Itanium ABI mangling scheme to its own v0 mangling scheme starting November 21, 2025. The new scheme preserves generic parameter information, produces more readable symbols for debuggers and profilers, and uses only alphanumeric characters for better platform compatibility. While symbols may be slightly larger, the change significantly improves debugging experience, especially for async code, closures, and generic functions. Users can revert to legacy mangling if needed using compiler flags.