E2. JustGRPO: back to left to right

E2. JustGRPO: back to left to right Lesson 18 of 19 in "The Flexibility Trap, from scratch". Paper: Ni et al., "The Flexibility Trap: Rethinking the Value of Arbitrary Order in Diffusion Language Models" (ICML 2026), https://arxiv.org/abs/2601.15165