Soft structural hints

Soft structural hints are opt-in. Set AUTOCONTEXT_SOFT_HINTS_ENABLED=true or hint_style=structural to ask the coach and curator to prefer constraints, invariants, verification checks, representations, and repair directions instead of route-locking solutions.

Persisted hint metadata records hint_style, support_evidence, is_structural, and route_prescriptive. A/B helpers summarize mean score, response length, novelty, rollback rate, and hint adoption rate by style.