Introduce reducer.Reducer interface and update the engine to use abstract
expr.Expression and reducer.Reducer types, enabling pluggable reduction
strategies.
- Add pkg/reducer/reducer.go with Reducer interface.
- Add pkg/lambda/reducer.go with NormalOrderReducer implementation.
- Update engine to accept expr.Expression and reducer.Reducer.
- Update plugins to use expr.Expression directly (no pointer dereference).
- Update main and tests to pass reducer to engine.
Closes#30
