Hack Day: Dummy Exchange + Real-Time Margin/Risk Controls

## Summary
Add a dummy exchange simulation and real-time margin/risk controls to enrich execution realism and controls.

## Jump-Off State(s)
- `009-order-management-matcher`

## Graph Intent
`canonical candidate`

## Scope
- Exchange simulator for matching/venue semantics.
- Account-level margin model and pre-trade risk checks.
- Real-time risk telemetry and breach actions.

## Acceptance Criteria
- Orders route through exchange simulation flow.
- Risk checks can block/limit executions based on policy.
- Risk events are observable in UI and telemetry stack.

## Decision Gate (Promote/Park)
Promote if learning value and system realism increase without unacceptable complexity.

Start at the `009-order-management-matcher` state and trace the existing order-routing flow before assessing the exchange simulator and account-level risk scope. Done means orders use the simulation flow, policy checks can block or limit executions, and risk events appear in the UI and telemetry stack.

Written by the indexing model from the issue text.