v0.2.0 - generated exercises & adaptive difficulty

• 8 exercise generator families: endless non-repeating variants with seeds baked into exercise ids (fully reproducible)
• Construction-time correctness: debugging variants prove their planted bug fails a test before they can exist; code-reading answers come from executing the snippet
• Information-optimal difficulty: each drill targets ~65% predicted success for your current rating
• First-run UX fixes from 0.1.1 included