feat(util): add GLib command stream for nonblocking line reads

Add a small GLib-backed helper for command stdout that integrates with the main loop instead of blocking on getline() in a worker thread.

The helper keeps the existing child setup semantics used by Waybar commands, including process groups, parent-death signaling, and WAYBAR_OUTPUT_NAME propagation.

This is the foundation for moving long-running custom commands away from manual poll/read logic in the module itself.

Signed-off-by: Austin Horstman <khaneliman12@gmail.com>

include/util/command_line_stream.hpp

@@ -0,0 +1,40 @@
+#pragma once
+
+#include <glibmm/main.h>
+#include <glibmm/spawn.h>
+
+#include <functional>
+#include <string>
+
+namespace waybar::util::command {
+
+class LineStream {
+ public:
+  using OutputCallback = std::function<void(const std::string&)>;
+  using ExitCallback = std::function<void(int)>;
+
+  LineStream(std::string output_name, OutputCallback on_output, ExitCallback on_exit);
+  ~LineStream();
+
+  void start(const std::string& cmd);
+  void stop();
+  bool running() const;
+
+ private:
+  bool handleStdout(Glib::IOCondition condition);
+  void handleExit(Glib::Pid pid, int status);
+  void closeStdout();
+  void drainStdout(bool flush_trailing_line);
+  static int statusToExitCode(int status);
+
+  std::string output_name_;
+  OutputCallback on_output_;
+  ExitCallback on_exit_;
+  std::string buffer_;
+  Glib::Pid pid_;
+  int stdout_fd_;
+  sigc::connection stdout_connection_;
+  sigc::connection child_connection_;
+};
+
+}  // namespace waybar::util::command