Hack Day: AuthN/AuthZ + Step-Up MFA for Trade Actions

## Summary
Add formal authentication and authorization, including step-up MFA for high-risk trade actions.

## Jump-Off State(s)
- `012-platform-convergence-c3`

## Graph Intent
`canonical candidate`

## Scope
- Identity/authN integration.
- Role/permission model for read-only vs trade operations.
- Step-up MFA policy for trade submit/cancel/force-fill.

## Acceptance Criteria
- Protected operations require authenticated principal + authorization checks.
- Step-up challenge applies to configured sensitive actions.
- Audit records capture actor, action, decision, and context.

## Decision Gate (Promote/Park)
Promote if security controls are robust and developer/operator workflow impact is acceptable.