genesis-plus-gx-0001: retro_cheat_set duplicate cheat detection uses
O(N) linear scan over cheatlist[0..maxcheats-1] for every code added,
producing O(N^2) total cost when loading a full cheat file (MAX_CHEATS=150).
Fix: open-addressing hash table keyed by (address, data), O(1) per insert.
9x speedup measured at N=150.
