fix: spawned clients were attaching to the host wayland display

another late night. testwm was launching terminals on the host compositor
because WAYLAND_DISPLAY was never pointed at our own socket. set it before
execlp. fixed, barely.

examples/testwm.cpp

@@ -50,6 +50,7 @@ struct Server {
 	struct wlr_keyboard* keyboard = nullptr;
 	wlr::SignalSink key_sink;
 	wl_event_source* scheduler_source = nullptr;
+	const char* socket_name = nullptr;
 
 	uint32_t mod = WLR_MODIFIER_ALT;
 	std::string terminal = "kitty";
@@ -123,6 +124,8 @@ static void cycle_focus(int dir) {
 static void spawn() {
 	pid_t p = fork();
 	if (p == 0) {
+		if (S->socket_name)
+			setenv("WAYLAND_DISPLAY", S->socket_name, 1);
 		execlp(S->terminal.c_str(), S->terminal.c_str(), nullptr);
 		_exit(127);
 	}
@@ -329,6 +332,7 @@ int main() {
 	S->scheduler_source = wl_event_loop_add_timer(S->display.event_loop(), frame_tick, nullptr);
 
 	const char* socket = S->display.add_socket_auto();
+	S->socket_name = socket;
 	if (!socket)
 		return 1;
 	fprintf(stderr,