Add undo system with coalescing logic and comprehensive tests.

- Implemented robust undo system supporting coalescing of text operations (insert, backspace, delete).
- Added `UndoSystem` integration into the editor/commands pipeline.
- Wrote extensive unit tests for various undo/redo scenarios, including multiline operations, cursor preservation, and history management.
- Refactored to ensure consistent cursor behavior during undo/redo actions.
- Updated CMake to include new tests.

tests/test_visual_line_mode.cc

@@ -122,4 +122,37 @@ TEST (VisualLineMode_CancelWithCtrlG)
 		std::cerr << "Got      (len=" << got.size() << ") bytes: " << dump_bytes(got) << "\n";
 	}
 	ASSERT_TRUE(got == exp);
+}
+
+
+TEST (Yank_ClearsMarkAndVisualLine)
+{
+	InstallDefaultCommands();
+
+	Editor ed;
+	ed.SetDimensions(24, 80);
+
+	Buffer b;
+	b.insert_text(0, 0, "foo\nbar\n");
+	b.SetCursor(1, 0);
+	ed.AddBuffer(std::move(b));
+
+	ASSERT_TRUE(ed.CurrentBuffer() != nullptr);
+	Buffer *buf = ed.CurrentBuffer();
+
+	// Seed mark + visual-line highlighting.
+	buf->SetMark(buf->Curx(), buf->Cury());
+	ASSERT_TRUE(buf->MarkSet());
+
+	ASSERT_TRUE(Execute(ed, std::string("visual-line-toggle")));
+	ASSERT_TRUE(Execute(ed, std::string("down"), std::string(), 1));
+	ASSERT_TRUE(buf->VisualLineActive());
+
+	// Yank should clear mark and any highlighting.
+	ed.KillRingClear();
+	ed.KillRingPush("X");
+	ASSERT_TRUE(Execute(ed, std::string("yank")));
+
+	ASSERT_TRUE(!buf->MarkSet());
+	ASSERT_TRUE(!buf->VisualLineActive());
 }