Function with_max_threads 

pub fn with_max_threads<R>(n: usize, f: impl FnOnce() -> R) -> R

Run f with the thread limit set to n on this thread only (0 = no override).

set_max_threads is process-wide, which is right for a command-line tool and wrong for a library host running several solves at once with different budgets. Every stage reads the limit on the thread that called the solver, and the solvers that hand work to their own threads pass the override on, so a solve inside f keeps to n threads whatever the rest of the process does. The previous value is restored when f returns or unwinds.