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.

SwapRecorder.h

@@ -0,0 +1,19 @@
+// SwapRecorder.h - minimal swap journal recording interface for Buffer mutations
+#pragma once
+
+#include <cstddef>
+#include <string_view>
+
+namespace kte {
+// SwapRecorder is a tiny, non-blocking callback interface.
+// Implementations must return quickly; Buffer calls these hooks after a
+// mutation succeeds.
+class SwapRecorder {
+public:
+	virtual ~SwapRecorder() = default;
+
+	virtual void OnInsert(int row, int col, std::string_view bytes) = 0;
+
+	virtual void OnDelete(int row, int col, std::size_t len) = 0;
+};
+} // namespace kte