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/tests/step1_test.cpp

@@ -0,0 +1,16 @@
+#include "../src/ast/Module.h"
+#include <cassert>
+#include <iostream>
+
+int main() {
+    Module m("SFE_M001", "SimpleFunctionExample", "python");
+
+    assert(m.id == "SFE_M001");
+    assert(m.name == "SimpleFunctionExample");
+    assert(m.targetLanguage == "python");
+    assert(m.conceptType == "Module");
+    assert(m.parent == nullptr);
+
+    std::cout << "Step 1: PASS" << std::endl;
+    return 0;
+}