Sprint 2 Step 1: base ASTNode class and Module concept

Adds the C++ editor stack foundation with ASTNode (id, conceptType, parent pointer, virtual dtor) and Module as the first concrete concept (name, targetLanguage). Header-only, compiles with C++20, test passes all assertions.

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

editor/src/ast/ASTNode.h

@@ -0,0 +1,11 @@
+#pragma once
+#include <string>
+
+class ASTNode {
+public:
+    std::string id;
+    std::string conceptType;
+    ASTNode* parent = nullptr;
+
+    virtual ~ASTNode() = default;
+};