Deliberate Wraparound

Implement unsigned int fnv1a_step(unsigned int h, unsigned char byte) — one FNV-1a round: h ^= byte; h *= 16777619u. Unsigned overflow is *defined* modular arithmetic; this hash depends on it. Then implement int mod16(int x) returning x mod 16 for non-negative x using only unsigned reasoning (no % on negative signed values).