Function surb_buffer_target_ceiling 

pub(crate) const fn surb_buffer_target_ceiling(rb_capacity: usize) -> usize

Largest SURB buffer target a Session may request, given the ring buffer capacity backing it.

Kept strictly below rb_capacity so the balancer has somewhere to overshoot into before the ring buffer starts overwriting. See SURB_BUFFER_TARGET_NUMERATOR.