Keep deterministic fallback output evidence-aware

[simaba]

Fallback-language correction

• softens fallback-only language that presented a reported input concern as a confirmed performance gap
• changes the deterministic Define-phase label from Problem confirmed to Reported problem statement
• keeps live-model and fallback provenance unchanged
• adds regression coverage for evidence-aware fallback wording

The legacy internal helper is not part of the supported engine path; the engine continues to use the provenance-aware assessment service.