Add scenarios.py: run the #952 demand↔supply engine under three key-rate
envelopes (base = hold_last_rate flat; conservative +2.0 п.п.; aggressive
−3.0 п.п., widening with horizon, clamped ≥0) → one ScenarioForecast each.
Single lever = rate_path into #952 (β stays counted-once in §9.4). Pure
build_rate_envelopes separated from DB orchestrator; advisory inherits #952
cap; graceful no-macro. Deterministic, no LLM, no SQL. 47 tests.
