Add swap journaling and group undo/redo with extensive tests.

- Introduced SwapManager for sidecar journaling of buffer mutations, with a safe recovery mechanism.
- Added group undo/redo functionality, allowing atomic grouping of related edits.
- Implemented `SwapRecorder` and integrated it as a callback interface for mutations.
- Added unit tests for swap journaling (save/load/replay) and undo grouping.
- Refactored undo to support group tracking and ID management.
- Updated CMake to include the new tests and swap journaling logic.

UndoNode.h

@@ -15,6 +15,7 @@ struct UndoNode {
 	UndoType type{};
 	int row{};
 	int col{};
+	std::uint64_t group_id = 0; // 0 means ungrouped; non-zero means undo/redo as an atomic group
 	std::string text;
 	UndoNode *parent = nullptr; // previous state; null means pre-first-edit
 	UndoNode *child  = nullptr; // next in current timeline