Keep deterministic fallback output evidence-aware

[simaba]

Fallback-language and API-boundary 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
• documents src.engine.run_assessment() as the only supported application-facing generation entry point
• adds a regression test that fails if the engine is ever wired back to the legacy phase-level generator
• adds regression coverage for evidence-aware fallback wording

The legacy helper remains private to the low-level phase module for now. It is explicitly outside the supported API boundary, so no supported execution path can bypass provenance or strict-mode behavior.