Step 303: New AST nodes — AsyncFunction, AwaitExpression, LambdaExpression, DecoratorAnnotation (12/12 tests)

4 modern language construct nodes with full JSON roundtrip serialization.
AsyncFunction extends Function with isAsync flag, AwaitExpression wraps
awaited expressions, LambdaExpression captures variables with captureList,
DecoratorAnnotation models @decorator(args) patterns.

Co-Authored-By: Claude Opus 4.6 <noreply@anthropic.com>

editor/src/ast/AsyncNodes.h

@@ -0,0 +1,57 @@
+#pragma once
+#include "ASTNode.h"
+#include "Function.h"
+#include "Expression.h"
+#include <string>
+#include <vector>
+
+// AsyncFunction — an async function (extends Function)
+class AsyncFunction : public Function {
+public:
+    bool isAsync = true;
+
+    AsyncFunction() { conceptType = "AsyncFunction"; isAsync = true; }
+    AsyncFunction(const std::string& id_, const std::string& name_)
+        : Function(id_, name_) {
+        this->conceptType = "AsyncFunction";
+        isAsync = true;
+    }
+};
+
+// AwaitExpression — await <expression>
+class AwaitExpression : public Expression {
+public:
+    AwaitExpression() { conceptType = "AwaitExpression"; }
+    AwaitExpression(const std::string& id_) {
+        this->id = id_;
+        this->conceptType = "AwaitExpression";
+    }
+    // Children: "expression" (the awaited expression)
+};
+
+// LambdaExpression — anonymous function / closure
+class LambdaExpression : public Expression {
+public:
+    std::vector<std::string> captureList;  // captured variables
+
+    LambdaExpression() { conceptType = "LambdaExpression"; }
+    LambdaExpression(const std::string& id_) {
+        this->id = id_;
+        this->conceptType = "LambdaExpression";
+    }
+    // Children: "parameters" (list of Parameter), "body" (list of Statement)
+};
+
+// DecoratorAnnotation — @decorator(args) on a function/class
+class DecoratorAnnotation : public ASTNode {
+public:
+    std::string name;
+
+    DecoratorAnnotation() { conceptType = "DecoratorAnnotation"; }
+    DecoratorAnnotation(const std::string& id_, const std::string& name_)
+        : name(name_) {
+        this->id = id_;
+        this->conceptType = "DecoratorAnnotation";
+    }
+    // Children: "arguments" (list of Expression)
+};