note: r7_v4 (Nano Banana 2) locked as B3 FINAL. Redo against new matplotlib A4 canonical: 3-tier bench (L3 at S wall), 20ft walls on 21ft slab with 6-inch lip, centered 16deg gable, heater in NE corner, frameless top-right glass corner. Supersedes r6_v1 lock from 2026-04-20.