Function promotion_backoff_blocks 

pub fn promotion_backoff_blocks(attempts: u8, check_interval_blocks: u32) -> u32

Compute the back-off in blocks to wait before the next promotion attempt after attempts consecutive failures. The first failure triggers a 1× wait, doubling each subsequent failure: 1×, 2×, 4×, 8×, 16×, 32× the check interval, with the shift saturated to keep multiplication safe.