PROTOTYPE — Scale Simulator, second Phase 2 module. Three mechanisms in one build: Digital, Dial (spring), and Balance Beam. Balance Beam is single-unit-system per question (real beam scales aren't dual-marked) with the other unit shown in the readout — not simultaneous dual-marked graphics like the Ruler/Dial. No drag-to-balance interaction yet — beam positions are shown static and read, not adjusted by hand. No hil-shell/session-logger yet.