Report both arithmetic and geometric mean returns

[Yuning598]

Background

The explorer currently reports a single mean monthly long-short return. The summary should report both arithmetic and geometric mean returns using clearly stated and internally consistent formulas.

Scope

• Retain the arithmetic mean of the selected monthly long-short series.
• Add the geometric mean based on compounded wealth over the same observations.
• State the reporting frequency explicitly; if annualized values are shown, annualize both measures consistently and label them separately.
• Recalculate both measures when the user changes the sample window.

Acceptance criteria

• Arithmetic and geometric mean returns are displayed as separate statistics.
• The UI identifies whether each statistic is monthly or annualized.
• The geometric calculation handles empty samples and returns at or below -100% safely.
• Python output and browser-side window calculations use matching definitions.
• Formula documentation and regression tests are included.