Add visual-line mode support with tests and UI integration.

- Introduced visual-line mode for multi-line selection and edits.
- Implemented commands, rendering, and keyboard shortcuts.
- Added tests for broadcast operations in visual-line mode.

CMakeLists.txt

@@ -302,6 +302,7 @@ if (BUILD_TESTS)
             tests/test_piece_table.cc
             tests/test_search.cc
             tests/test_reflow_paragraph.cc
+            tests/test_visual_line_mode.cc
 
             # minimal engine sources required by Buffer
             PieceTable.cc