Teach risk rules and stateful strategy behavior #114

Affected library: backtest
Current version: 0.1.8
Location: risk and stateful-strategy tutorial

Problem

Readers cannot see how stops, exits, portfolio drawdown reduction, and strategy state interact across callbacks and fills.

Required outcome

Build a runnable tutorial showing causal risk-event order, the resulting stop and reduction fills, and a strategy that needs state between callbacks. Connect it to the risk-management guide, the existing stateful example, and relevant book material.

Acceptance criteria

• Executed results show the expected stop and reduction fills, fraction, and exposure change.
• A repeated breach and a later eligible risk event produce the documented behavior.
• The page explains the order of marks, checks, decisions, and fills using the observed records.
• Installed-wheel checks and the strict MkDocs build pass.

Depends on the portfolio-reduction fix and shared tutorial inputs.