fix(ai): release horn when human driver takes over from AI

tests/unit/engine_stubs.cpp

@@ -0,0 +1,81 @@
+// stand-ins for the parts of the engine which the core library (eu07_core) links against, but which the
+// unit tests don't exercise. when a change makes the library use something new from the engine and the
+// test build fails to link, either move the real source into the library (CORE_SOURCES in the main
+// CMakeLists.txt, if it stands on its own) or add a stub here.
+
+#include "stdafx.h"
+#include "utilities/Globals.h"
+#include "utilities/Logs.h"
+#include "scene/scenenodegroups.h"
+#include "scene/scenelayers.h"
+
+#include <cstdlib>
+#include <iostream>
+
+// settings ---------------------------------------------------------------------------------------
+
+global_settings &GetGlobalSettings() {
+	// defaults, as before reading eu07.ini
+	static global_settings settings;
+	return settings;
+}
+
+// logs: quiet, unless EU07_UNIT_TESTS_LOG is set --------------------------------------------------
+
+namespace {
+bool log_enabled() {
+	static bool const enabled{std::getenv("EU07_UNIT_TESTS_LOG") != nullptr};
+	return enabled;
+}
+} // namespace
+
+void WriteLog(const char *str, logtype const, bool const isError) {
+	if (log_enabled() && str != nullptr) {
+		(isError ? std::cerr : std::cout) << str << "\n";
+	}
+}
+
+void WriteLog(const std::string &str, logtype const Type) {
+	WriteLog(str.c_str(), Type, false);
+}
+
+void ErrorLog(const std::string &str, logtype const Type) {
+	WriteLog(str.c_str(), Type, true);
+}
+
+// scenery structure, which the parser reports include directives to --------------------------------
+// the declarations come from the engine's headers, the stubs can't change them (hence the NOSONARs)
+
+namespace scene {
+
+node_groups Groups; // NOSONAR the engine's extern global, scene/scenenodegroups.h
+node_layers Layers; // NOSONAR the engine's extern global, scene/scenelayers.h
+
+group_handle node_groups::create(bool const) { // NOSONAR signature from scene/scenenodegroups.h
+	return null_handle;
+}
+
+group_handle node_groups::close() { // NOSONAR signature from scene/scenenodegroups.h
+	return null_handle;
+}
+
+layer_handle node_layers::open(std::string const &, include_site) { // NOSONAR signature from scene/scenelayers.h
+	return null_handle;
+}
+
+layer_handle node_layers::close() { // NOSONAR signature from scene/scenelayers.h
+	return null_handle;
+}
+
+void node_layers::instance_begin(std::string const &, source_span const &) { // NOSONAR signature from scene/scenelayers.h
+	// no layers in the tests, so nothing to keep track of
+}
+
+// basic_node has no key function for the Itanium C++ ABI (tooltip() is declared plain but defined inline), so
+// GCC and Clang emit its vtable wherever scenenode.h is used, here too, and the vtable wants radius_().
+// the same as the game's, in scene/scenenode.cpp
+float basic_node::radius_() {
+	return 0.f;
+}
+
+} // namespace scene