fix(cpp): emit generic_arg references for base-class template arguments

The C++ base_class_clause handler's `template_type` branch read the base
name (`sub.child_by_field_name("name")`) and emitted the `inherits` edge,
but never descended into the base's `template_argument_list`. As a result
`class Car : public Base<Dep>` emitted `Car -> Base` (inherits) yet dropped
the `Car -> Dep` generic_arg reference entirely.

The Java handler `_emit_java_parent_type` already emits these generic_arg
references for base-class type arguments; C++ was the asymmetric gap.

Fix: after emitting the `inherits` edge, grab the base's `arguments` field
(the `template_argument_list`) and run `_cpp_collect_type_refs` over each
named argument with the generic flag set, emitting a `references` edge
(context "generic_arg") per collected type, guarding target != class node.
`_cpp_collect_type_refs` already handles nested/qualified args, so
`Base<std::vector<Dep>>` is covered too.

Adds a templated base (`Connection<T>`) + derived class
(`PooledClient : public Connection<HttpClient>`) to tests/fixtures/sample.cpp
and a test mirroring the Java generic-parents test.

tests/fixtures/sample.cpp

@@ -36,6 +36,17 @@ struct RetryingHttpClient : HttpClient {
     int maxRetries;
 };
 
+template <typename T>
+class Connection {
+public:
+    T resource;
+};
+
+class PooledClient : public Connection<HttpClient> {
+public:
+    int poolSize;
+};
+
 int main() {
     HttpClient client("https://api.example.com");
     std::string response = client.get("/users");