Implement Coppock Curve Strategy

[cinar]

Implement the Coppock Curve trading strategy using the existing momentum.CoppockCurve indicator.

Location: strategy/momentum/coppock_curve_strategy.go

Strategy Rules:

• Buy Action: Coppock Curve crosses above zero (long-term buying signal).
• Sell Action: Coppock Curve crosses below zero.
• Hold Action: Otherwise.

Implementation Details:

• Follow the strategy.Strategy interface.
• Use momentum.NewCoppockCurve() or allow custom parameters.
• Provide unit tests with historical data in strategy/momentum/testdata/coppock_curve_strategy.csv.
• Ensure 100% test coverage.