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/ASTNode.h

@@ -1,5 +1,7 @@
 #pragma once
 #include <string>
+#include <vector>
+#include <map>
 
 class ASTNode {
 public:
@@ -8,4 +10,50 @@ public:
     ASTNode* parent = nullptr;
 
     virtual ~ASTNode() = default;
+
+    // Multi-valued child: append to role
+    void addChild(const std::string& role, ASTNode* child) {
+        child->parent = this;
+        children_[role].push_back(child);
+    }
+
+    // Single-valued child: set (replaces if exists)
+    void setChild(const std::string& role, ASTNode* child) {
+        auto& vec = children_[role];
+        if (!vec.empty()) {
+            vec[0]->parent = nullptr;
+            vec[0] = child;
+        } else {
+            vec.push_back(child);
+        }
+        child->parent = this;
+    }
+
+    // Get single-valued child (nullptr if not set)
+    ASTNode* getChild(const std::string& role) const {
+        auto it = children_.find(role);
+        if (it != children_.end() && !it->second.empty()) {
+            return it->second[0];
+        }
+        return nullptr;
+    }
+
+    // Get multi-valued children (empty vector if role not present)
+    const std::vector<ASTNode*>& getChildren(const std::string& role) const {
+        static const std::vector<ASTNode*> empty;
+        auto it = children_.find(role);
+        return it != children_.end() ? it->second : empty;
+    }
+
+    // Get all children across all roles
+    std::vector<ASTNode*> allChildren() const {
+        std::vector<ASTNode*> result;
+        for (const auto& [role, kids] : children_) {
+            result.insert(result.end(), kids.begin(), kids.end());
+        }
+        return result;
+    }
+
+private:
+    std::map<std::string, std::vector<ASTNode*>> children_;
 };