docs: add architecture tour for crate ownership and event flow

[Ashutosh0x]

Create a contributor-oriented architecture tour under docs/architecture/ that explains the workspace at a practical level.

Acceptance criteria:

• Explain the main event flow: ingestion -> event bus -> features/signals -> strategy -> risk -> execution/OMS -> UI/persistence.
• Include a crate ownership table matching CONTRIBUTING.md.
• Link from README or docs/architecture/architecture_overview.md.
• Keep claims factual and avoid production-trading promises.