`skyscraper/fp-rounding/src/rounding_guard`: Force one per thread.

[recmo]

On 2025-08-27 in d91b2f2 “Merge pull request #142 from worldfnd/sl/skyscraper-refactor”:

Force one per thread.

///
/// # Safety
///
/// This type is marked !Send + !Sync because FPCR is a per-core / per OS-thread
/// register.
// TODO: Force one per thread.
pub struct RoundingGuard<M: RoundingDirectionMarker> {
    previous: RoundingDirection,
    mode:     PhantomData<M>,
    _marker:  NotSendSync,
}

From skyscraper/fp-rounding/src/rounding_guard.rs:20