refactor: rename interpreter to runtime and use receiver methods

- Rename pkg/interpreter to pkg/runtime
- Move ReduceOnce to new pkg/normalorder package
- Convert standalone functions (Substitute, Rename, GetFree, IsFree)
  to receiver methods on concrete expression types
- Change Set from pointer receivers to value receivers
- Update all references from interpreter to runtime terminology

Co-Authored-By: Claude Opus 4.5 <noreply@anthropic.com>

pkg/runtime/events.go

@@ -0,0 +1,13 @@
+package runtime
+
+// 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
+)