Add TestFrontend documentation and UndoSystem buffer reference update.

- Document `TestFrontend` for programmatic testing, including examples and usage details.
- Add `UpdateBufferReference` to `UndoSystem` to support updating buffer associations.

test_undo.cc

@@ -0,0 +1,102 @@
+#include <iostream>
+#include <cassert>
+#include <fstream>
+#include "Editor.h"
+#include "TestFrontend.h"
+#include "Command.h"
+#include "Buffer.h"
+
+
+int
+main()
+{
+	// Install default commands
+	InstallDefaultCommands();
+
+	Editor editor;
+	TestFrontend frontend;
+
+	// Initialize frontend
+	if (!frontend.Init(editor)) {
+		std::cerr << "Failed to initialize frontend\n";
+		return 1;
+	}
+
+	// Create a temporary test file
+	std::string err;
+	const char *tmpfile = "/tmp/kte_test_undo.txt";
+	{
+		std::ofstream f(tmpfile);
+		if (!f) {
+			std::cerr << "Failed to create temp file\n";
+			return 1;
+		}
+		f << "\n"; // Write one newline so file isn't empty
+		f.close();
+	}
+
+	if (!editor.OpenFile(tmpfile, err)) {
+		std::cerr << "Failed to open test file: " << err << "\n";
+		return 1;
+	}
+
+	Buffer *buf = editor.CurrentBuffer();
+	assert(buf != nullptr);
+
+	// Initialize cursor to (0,0) explicitly
+	buf->SetCursor(0, 0);
+
+	std::cout << "test_undo: Testing undo/redo system\n";
+	std::cout << "====================================\n\n";
+
+	bool running = true;
+
+	// Test 1: Insert text and verify buffer contains expected text
+	std::cout << "Test 1: Insert text 'Hello'\n";
+	frontend.Input().QueueText("Hello");
+
+	while (!frontend.Input().IsEmpty() && running) {
+		frontend.Step(editor, running);
+	}
+
+	assert(buf->Rows().size() >= 1);
+	std::string line_after_insert = std::string(buf->Rows()[0]);
+	assert(line_after_insert == "Hello");
+	std::cout << "  Buffer content: '" << line_after_insert << "'\n";
+	std::cout << "  ✓ Text insertion verified\n\n";
+
+	// Test 2: Undo insertion - text should be removed
+	std::cout << "Test 2: Undo insertion\n";
+	frontend.Input().QueueCommand(CommandId::Undo);
+
+	while (!frontend.Input().IsEmpty() && running) {
+		frontend.Step(editor, running);
+	}
+
+	assert(buf->Rows().size() >= 1);
+	std::string line_after_undo = std::string(buf->Rows()[0]);
+	assert(line_after_undo == "");
+	std::cout << "  Buffer content: '" << line_after_undo << "'\n";
+	std::cout << "  ✓ Undo successful - text removed\n\n";
+
+	// Test 3: Redo insertion - text should be restored
+	std::cout << "Test 3: Redo insertion\n";
+	frontend.Input().QueueCommand(CommandId::Redo);
+
+	while (!frontend.Input().IsEmpty() && running) {
+		frontend.Step(editor, running);
+	}
+
+	assert(buf->Rows().size() >= 1);
+	std::string line_after_redo = std::string(buf->Rows()[0]);
+	assert(line_after_redo == "Hello");
+	std::cout << "  Buffer content: '" << line_after_redo << "'\n";
+	std::cout << "  ✓ Redo successful - text restored\n\n";
+
+	frontend.Shutdown();
+
+	std::cout << "====================================\n";
+	std::cout << "All tests passed!\n";
+
+	return 0;
+}