Sprint 2 Step 2: child links, Function and Variable concepts

Adds generic child link support to ASTNode (addChild/setChild for multi/single-
valued roles, getChildren/getChild/allChildren for traversal). Function and
Variable as first child concepts of Module. Test builds Module->Function tree,
walks parent/child pointers in both directions, verifies single-valued replace.

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

editor/src/ast/Variable.h

@@ -0,0 +1,14 @@
+#pragma once
+#include "ASTNode.h"
+
+class Variable : public ASTNode {
+public:
+    std::string name;
+
+    Variable() { conceptType = "Variable"; }
+    Variable(const std::string& id, const std::string& name)
+        : name(name) {
+        this->id = id;
+        this->conceptType = "Variable";
+    }
+};