Replace ad-hoc JSON parsing with a strict Pydantic model
(TrainingExample with typed OutputPair). All data loading goes
through load_examples() which fails loudly on invalid data.
- Convert v3_structured.jsonl from "searches" to "output" format
- Rewrite all consumer scripts (prepare, validate, score, analyze)
to load through the Pydantic schema
- Prepared train/val files are ephemeral build artifacts
- Restore LFM2 and GEPA experiments under experiments/
- Add pydantic>=2.0 to dependencies
