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/runtime.go

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