Add ReflowUndo tests and integrate InsertRow undo support

- Added `test_reflow_undo.cc` to validate undo/redo workflows for reflow operations.
- Introduced `UndoType::InsertRow` in `UndoSystem` for tracking row insertion changes in undo history.
- Updated `UndoNode.h` and `UndoSystem.cc` to support row insertion as a standalone undo step.
- Enhanced reflow paragraph functionality to properly record undo/redo actions for both row deletion and insertion.
- Enabled legacy/extended undo tests in `test_undo.cc` for comprehensive validation.
- Updated `CMakeLists.txt` to include new test file in the build target.

tests/test_reflow_undo.cc

@@ -0,0 +1,69 @@
+#include "Test.h"
+
+#include "Buffer.h"
+#include "Command.h"
+#include "Editor.h"
+#include "UndoSystem.h"
+
+#include <string>
+
+
+static std::string
+to_string_rows(const Buffer &buf)
+{
+	std::string out;
+	for (const auto &r: buf.Rows()) {
+		out += static_cast<std::string>(r);
+		out.push_back('\n');
+	}
+	return out;
+}
+
+
+TEST (ReflowUndo)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	const std::string initial =
+		"This is a very long line that should be reflowed into multiple lines to see if undo works correctly.\n";
+	b.insert_text(0, 0, initial);
+	b.SetCursor(0, 0);
+
+	// Commit initial insertion so it's its own undo step
+	if (auto *u = b.Undo())
+		u->commit();
+
+	ed.AddBuffer(std::move(b));
+
+	Buffer *buf = ed.CurrentBuffer();
+	ASSERT_TRUE(buf != nullptr);
+
+	const std::string original_dump = to_string_rows(*buf);
+
+	// Reflow with small width
+	const int width = 20;
+	ASSERT_TRUE(Execute(ed, "reflow-paragraph", "", width));
+
+	const std::string reflowed_dump = to_string_rows(*buf);
+	ASSERT_TRUE(reflowed_dump != original_dump);
+	ASSERT_TRUE(buf->Rows().size() > 1);
+
+	// Undo reflow
+	ASSERT_TRUE(Execute(ed, "undo", "", 1));
+	const std::string after_undo_dump = to_string_rows(*buf);
+
+	if (after_undo_dump != original_dump) {
+		fprintf(stderr, "Undo failed.\nExpected:\n%s\nGot:\n%s\n", original_dump.c_str(),
+		        after_undo_dump.c_str());
+	}
+	EXPECT_TRUE(after_undo_dump == original_dump);
+
+	// Redo reflow
+	ASSERT_TRUE(Execute(ed, "redo", "", 1));
+	const std::string after_redo_dump = to_string_rows(*buf);
+	EXPECT_TRUE(after_redo_dump == reflowed_dump);
+}