V9 + DU Stage 1 integration. The ATA 24 electrical model remains authoritative and drives the five-DU ATA 31 workspace. The DU page now also includes source-backed ATA 21/36 AIR/bleed and ATA 24-70 Circuit Breaker applications; engine/APU source availability follows the parent state while standalone AIR/CB controls remain local unless the parent explicitly overrides them. This is training software, not approved operational software.