Function build_cycle_walk_state 

pub fn build_cycle_walk_state(
    range: u64,
    seed: u64,
    stream: u64,
) -> CycleWalkState

Joint Feistel-state derivation. Single-call construction-time invocation per node instance; the macro emits the call inside the generated CycleWalk::new(range, seed, stream).

Panics if range is 0 — preserves the construction-time validation contract from the pre-Phase-E hand-written form.