Phase 4 complete: level2 dataset, distractor env, 90 tests passing

fix: exponential backoff on rate limit (10 retries, 30s*attempt)

fix: retry on rate limit in generate_distractors, 21s sleep for free-tier 3 RPM

fix: propagate fatal API errors, strip markdown fences, cleanup

fix: count skipped rows toward progress interval

fix: sleep after error path, print progress every 10 iterations

feat: generate_distractors.py — GPT-4o-mini Level 2 dataset script

Phase 1 complete: schemas, reward design, project scaffold