Implement the Fisher Transform trading strategy using the existing momentum.Fisher indicator.

Location: strategy/momentum/fisher_strategy.go

Strategy Rules:

• Buy Action: Fisher Transform value crosses above its signal line (which is a 1-period lagged Fisher value).
• Sell Action: Fisher Transform value crosses below its signal line.
• Hold Action: Otherwise.

Implementation Details:

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