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.

TestFrontend.cc

@@ -0,0 +1,34 @@
+#include "TestFrontend.h"
+#include "Editor.h"
+#include "Command.h"
+#include <iostream>
+
+
+bool
+TestFrontend::Init(Editor &ed)
+{
+	ed.SetDimensions(24, 80);
+	return true;
+}
+
+
+void
+TestFrontend::Step(Editor &ed, bool &running)
+{
+	MappedInput mi;
+	if (input_.Poll(mi)) {
+		if (mi.hasCommand) {
+			Execute(ed, mi.id, mi.arg, mi.count);
+		}
+	}
+
+	if (ed.QuitRequested()) {
+		running = false;
+	}
+
+	renderer_.Draw(ed);
+}
+
+
+void
+TestFrontend::Shutdown() {}