Live audit for the GBA defect class across every classic platform: gpgx
switches sizes per system correctly (Genesis/SMS/GG); 7800's 64KB is the
documented whole-bus view (verified offset==CPU address with live data);
MSX's 512KB is real mapper RAM (verified nonzero post-boot). GB HRAM +
Genesis Z80 RAM already have dedicated regions. GBA IWRAM was the only
instance of the class.