Function round_kernel 

pub fn round_kernel(state: RoundState, round_constant: b32) -> RoundState

Executes exactly one SHA-256 round and advances the rolling schedule.

round_constant is a separate input so this kernel is an isolated Vivado resource/timing target. Pipeline stages connect it to a compile-time member of ROUND_CONSTANTS_RHDL. All additions are 32-bit modular additions.