Function init_rayon_for_script_verification 

pub fn init_rayon_for_script_verification()

Initialize Rayon thread pool for script verification.

Call this at node startup before any block validation.

• When script_verification_threads > 0: use that value explicitly.
• When 0: let Rayon use its default (respects RAYON_NUM_THREADS env; typically num_cpus). IBD scripts set RAYON_NUM_THREADS=nproc-1 for par-1 workers.

Only takes effect once per process.