Add Sprint 2 market analytics metrics

[BytecodeBrewer]

Goal

Add the core Sprint 2 market analytics metrics for historical exchange-rate data.

Why

After replacing mock data with a real historical data client, ARGUS needs stronger metrics to turn raw time-series data into useful market insights.

This ticket adds the first meaningful analytics layer for Sprint 2.

Scope

Implement the following metrics:

• cumulative return
• strongest and weakest daily movement
• rolling volatility

The metrics should be reusable, testable and independent from GUI code.

Acceptance criteria

• Cumulative return can be calculated from historical time-series data
• Strongest daily movement can be identified
• Weakest daily movement can be identified
• Rolling volatility can be calculated with a configurable window
• Metric functions are independent from GUI code
• Tests cover normal, flat and edge-case data
• Metric assumptions are documented if needed
• Existing tests still pass

Note

Priority: Must