refactor: make lambda expression types immutable (#38)

## Summary

- Change Abstraction, Application, and Variable to use private fields with getter methods.
- Return value types instead of pointers from constructors.
- Update all type switches to match value types instead of pointer types.

## Test plan

- [x] All existing tests pass (`make test`).

Reviewed-on: #38
Co-authored-by: M.V. Hutz <git@maximhutz.me>
Co-committed-by: M.V. Hutz <git@maximhutz.me>

pkg/interpreter/interpreter.go

@@ -0,0 +1,27 @@
+// Package interpreter provides the abstract Reducer interface for all expression
+// reduction strategies.
+package interpreter
+
+import (
+	"git.maximhutz.com/max/lambda/pkg/emitter"
+	"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.
+//
+// Reducers also implement the Emitter interface to allow plugins to observe
+// reduction lifecycle events (Start, Step, Stop).
+type Interpreter interface {
+	emitter.Emitter[Event]
+
+	// Run a single step. Returns whether the interpreter is complete or not.
+	Step() bool
+
+	// Run until completion.
+	Run()
+
+	// Expression returns the current expression state.
+	Expression() expr.Expression
+}