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`.

TestInputHandler.h

@@ -0,0 +1,33 @@
+/*
+ * TestInputHandler.h - programmable input handler for testing
+ */
+#ifndef KTE_TEST_INPUT_HANDLER_H
+#define KTE_TEST_INPUT_HANDLER_H
+
+#include "InputHandler.h"
+#include <queue>
+
+
+class TestInputHandler : public InputHandler {
+public:
+	TestInputHandler() = default;
+
+	~TestInputHandler() override = default;
+
+	bool Poll(MappedInput &out) override;
+
+	void QueueCommand(CommandId id, const std::string &arg = "", int count = 0);
+
+	void QueueText(const std::string &text);
+
+
+	bool IsEmpty() const
+	{
+		return queue_.empty();
+	}
+
+private:
+	std::queue<MappedInput> queue_;
+};
+
+#endif // KTE_TEST_INPUT_HANDLER_H