Function frontier_queue_len_init 

pub fn frontier_queue_len_init(queue_len: &str) -> Program

Build a GPU program that initializes the active queue length scalar.

This replaces a per-wave host-to-device zero upload in resident sparse traversal pipelines. Keeping initialization as a separate single-lane device step avoids the global-synchronization race that would occur if the multi-workgroup compaction kernel tried to clear and atomically increment the same scalar.