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`.

TestRenderer.h

@@ -0,0 +1,35 @@
+/*
+ * TestRenderer.h - minimal renderer for testing (no actual display)
+ */
+#ifndef KTE_TEST_RENDERER_H
+#define KTE_TEST_RENDERER_H
+
+#include "Renderer.h"
+#include <cstddef>
+
+
+class TestRenderer : public Renderer {
+public:
+	TestRenderer() = default;
+
+	~TestRenderer() override = default;
+
+	void Draw(Editor &ed) override;
+
+
+	std::size_t GetDrawCount() const
+	{
+		return draw_count_;
+	}
+
+
+	void ResetDrawCount()
+	{
+		draw_count_ = 0;
+	}
+
+private:
+	std::size_t draw_count_ = 0;
+};
+
+#endif // KTE_TEST_RENDERER_H