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/events.go

@@ -0,0 +1,13 @@
+package interpreter
+
+// Event represents lifecycle events during interpretation.
+type Event int
+
+const (
+	// StartEvent is emitted before interpretation begins.
+	StartEvent Event = iota
+	// StepEvent is emitted after each interpretation step.
+	StepEvent
+	// StopEvent is emitted after interpretation completes.
+	StopEvent
+)