Remove packaging.cmake, deprecate test_undo setup, and add new testing infrastructure.

- Delete `packaging.cmake` to streamline build system.
- Deprecate `test_undo` in CMake setup; condition builds on `BUILD_TESTS`.
- Introduce `TestFrontend`, `TestRenderer`, and `TestInputHandler` for structured testing.
- Update `GUIInputHandler` and `Command` for enhanced buffer save handling and overwrite confirmation.
- Enhance kill ring operations and new prompt workflows in `Editor`.

TestFrontend.h

@@ -0,0 +1,41 @@
+/*
+ * TestFrontend.h - headless frontend for testing with programmable input
+ */
+#ifndef KTE_TEST_FRONTEND_H
+#define KTE_TEST_FRONTEND_H
+
+#include "Frontend.h"
+#include "TestInputHandler.h"
+#include "TestRenderer.h"
+
+
+class TestFrontend final : public Frontend {
+public:
+	TestFrontend() = default;
+
+	~TestFrontend() override = default;
+
+	bool Init(Editor &ed) override;
+
+	void Step(Editor &ed, bool &running) override;
+
+	void Shutdown() override;
+
+
+	TestInputHandler &Input()
+	{
+		return input_;
+	}
+
+
+	TestRenderer &Renderer()
+	{
+		return renderer_;
+	}
+
+private:
+	TestInputHandler input_{};
+	TestRenderer renderer_{};
+};
+
+#endif // KTE_TEST_FRONTEND_H