refactor: extract Reducer interface and update engine to use abstractions

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

internal/plugins/debug.go

@@ -19,5 +19,5 @@ func NewLogs(logger *slog.Logger, process *engine.Engine) *Logs {
 }
 
 func (t *Logs) Step() {
-	t.logger.Info("reduction", "tree", (*t.process.Expression).String())
+	t.logger.Info("reduction", "tree", t.process.Expression.String())
 }