feat: add arbitrage phase a schema

[BobbyAxerol]

Summary

Describe what changed and why.

Type

• Bug fix
• Feature
• Engine/accounting change
• Nautilus parity change
• Documentation
• Tests

Backtest Assumptions

If this changes engine behavior, document:

• signal contract:
• sizing mode:
• execution/fill timing:
• fee/slippage/funding model:
• margin/leverage/liquidation behavior:
• portfolio/netting behavior:

Tests

Paste the commands run:

Checklist

• This PR targets dev.
• I did not commit directly to main.
• The change is scoped to one topic.
• Tests or reproducible validation are included.
• Documentation/examples are updated if public behavior changed.
• No unrelated files are included.