Refactor priority (maintainer)

This is an opinionated priority list for improving correctness and maintainability.

1. Artifact correctness first: trace validity, taxonomy correctness, schema discipline.
2. Single adapter interface: reduce backend complexity behind one contract.
3. Trace richness: phase-level entries and better diagnostics.
4. Ergonomics: CLI/API parity, clearer configuration surface.
5. Performance: only after correctness is boring.

Avoid polishing UX while the contract is still moving.