Implement branching undo system with tests and updates.

- Added branching model for undo/redo, enabling multiple redo paths and branch selection.
- Updated `UndoNode` to include `parent` and refined hierarchical navigation.
- Extended `UndoSystem` with branching logic for redo operations, supporting sibling branch selection.
- Overhauled tests to validate branching behavior and tree invariants.
- Refined editor command logic for undo/redo with repeat counts and branch selection.
- Enabled test-only introspection hooks for undo tree validation.
- Updated CMake to include test definitions (`KTE_TESTS` flag).

UndoNode.h

@@ -16,6 +16,7 @@ struct UndoNode {
 	int row{};
 	int col{};
 	std::string text;
-	UndoNode *child = nullptr; // next in current timeline
-	UndoNode *next  = nullptr; // redo branch
+	UndoNode *parent = nullptr; // previous state; null means pre-first-edit
+	UndoNode *child  = nullptr; // next in current timeline
+	UndoNode *next   = nullptr; // redo branch
 };