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

pkg/reducer/reducer.go

@@ -0,0 +1,15 @@
+// Package reducer provides the abstract Reducer interface for all expression
+// reduction strategies.
+package reducer
+
+import "git.maximhutz.com/max/lambda/pkg/expr"
+
+// Reducer defines the interface for expression reduction strategies.
+// Different evaluation modes (normal order, applicative order, SKI combinators,
+// etc.) implement this interface with their own reduction logic.
+type Reducer interface {
+	// Reduce performs all reduction steps on the expression, calling onStep
+	// after each reduction.
+	// Returns the final reduced expression.
+	Reduce(e expr.Expression, onStep func(expr.Expression)) expr.Expression
+}