Function assign_bucket

pub fn assign_bucket(subscriber_id: &str, seed: &str) -> u32

Deterministic bucket assignment: sha256("{subscriber_id}:{seed}"), first 4 bytes read big-endian, mod BUCKET_COUNT. Must match the TS implementation byte-for-byte (same hash input, same byte order, same modulo) — this is the cross-language bucketing contract, verified against packages/shared/src/experiments/bucketing-vectors.json below.