feat: add encoder measurement script (fixed ROC-AUC direction)

feat: Stage 3 InfoNCE structural adversarial - ROC-AUC=0.994, gap=1.20

feat: add Stage 2 InfoNCE training script

feat: Stage 2 InfoNCE fine-tuning - 50K samples, 3 epochs, energy_gap ~1.05

feat: add ExecutionEncoder source

feat: JEPA Stage 1 ExecutionEncoder - VICReg 50K training (5 epochs, MPS)

initial commit