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/tests/step2_test.cpp

@@ -0,0 +1,73 @@
+#include "../src/ast/Module.h"
+#include "../src/ast/Function.h"
+#include "../src/ast/Variable.h"
+#include <cassert>
+#include <iostream>
+
+int main() {
+    // Build tree: Module -> [Function, Function, Variable]
+    Module mod("M001", "Calculator", "python");
+    Function add("F001", "add");
+    Function subtract("F002", "subtract");
+    Variable pi("V001", "pi");
+
+    mod.addChild("functions", &add);
+    mod.addChild("functions", &subtract);
+    mod.addChild("variables", &pi);
+
+    // Verify parent pointers
+    assert(add.parent == &mod);
+    assert(subtract.parent == &mod);
+    assert(pi.parent == &mod);
+    assert(mod.parent == nullptr);
+
+    // Verify children by role
+    const auto& functions = mod.getChildren("functions");
+    assert(functions.size() == 2);
+    assert(functions[0] == &add);
+    assert(functions[1] == &subtract);
+
+    const auto& variables = mod.getChildren("variables");
+    assert(variables.size() == 1);
+    assert(variables[0] == &pi);
+
+    // Empty role returns empty vector
+    const auto& empty = mod.getChildren("annotations");
+    assert(empty.empty());
+
+    // Walk down and back up
+    for (auto* child : mod.getChildren("functions")) {
+        assert(child->parent == &mod);
+        assert(child->parent->conceptType == "Module");
+        auto* fn = static_cast<Function*>(child);
+        assert(!fn->name.empty());
+    }
+
+    // Verify names through downcast
+    assert(static_cast<Function*>(functions[0])->name == "add");
+    assert(static_cast<Function*>(functions[1])->name == "subtract");
+    assert(static_cast<Variable*>(variables[0])->name == "pi");
+
+    // allChildren returns everything
+    auto all = mod.allChildren();
+    assert(all.size() == 3);
+
+    // Single-valued child (setChild / getChild)
+    Function entry("F003", "main");
+    mod.setChild("entryPoint", &entry);
+    assert(mod.getChild("entryPoint") == &entry);
+    assert(entry.parent == &mod);
+
+    // Replace single-valued child
+    Function alt("F004", "alt_main");
+    mod.setChild("entryPoint", &alt);
+    assert(mod.getChild("entryPoint") == &alt);
+    assert(alt.parent == &mod);
+    assert(entry.parent == nullptr);  // old child's parent cleared
+
+    // getChild on unset role returns nullptr
+    assert(mod.getChild("nonexistent") == nullptr);
+
+    std::cout << "Step 2: PASS" << std::endl;
+    return 0;
+}