refactor: add window::stop() and remove quit flag

Added `window::stop()` (sets GLFW close flag). Updated
signal/timer handlers and Q key check to call it instead of
using a separate `quit` bool in main().

This encapsulates the close state in the window class (no
more external flag + manual checks). The render loop is now
simpler.

(The process_signals lambda and ASIO duration timer are
retained.)

cbt/window.cpp

@@ -83,4 +83,10 @@ auto window::raw() const -> GLFWwindow* {
     return m_window;
 }
 
+auto window::stop() -> void {
+    if (m_window) {
+        glfwSetWindowShouldClose(m_window, GLFW_TRUE);
+    }
+}
+
 }