Add benchmark-aware backtest metrics with pair tests

[TheoGoulart333]

Summary

Adds a buy-and-hold benchmark and strategy excess return to the backtesting
report.

Progresses #3

Pairing status

• @TheoGoulart333: implement benchmark and excess-return metrics.
• @AlterRemz: add deterministic tests for rising, falling, and flat
  markets; review calculation edge cases.
• Pair-review the completed implementation.
• Add a valid Co-authored-by trailer only after both contributors have
  made substantive contributions.

Current validation

• Existing project structure preserved.
• No future data is used; the benchmark compares the first and last close.
• No README content changed.
• New benchmark behavior covered by tests.
• CI passes with both contributors' work.

This PR intentionally remains a draft until the collaborator's tests and
review are included.