Design explicit scripted shock and regime scenarios

[Tim55667757]

Parent Feature: #43
Parent roadmap: #28

Scope

Provide deterministic or stochastic stress scenarios for trading-system QA when the use case requires shocks/regimes.

Acceptance criteria

• each scenario type maps to a test objective;
• shock/regime parameters have precise semantics;
• seeded behavior is reproducible where stochastic;
• market-data invariants remain valid;
• scenarios are not marketed as empirical forecasts.