follow stab55_11, 1. revalidate formal c64/c128 MG-vs-MG ratios after builder optimization; 2. record c64-l3 1.9898x, c128 small 12.3768x, c128 medium 3.2332x with passing residuals; 3. add per-side BiCGStab distortion checks and isolate single-coarse-block CPU staging as a capacity experiment. [opencode].