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).

CMakeLists.txt

@@ -319,6 +319,9 @@ if (BUILD_TESTS)
             ${SYNTAX_SOURCES}
     )
 
+    # Allow test-only introspection hooks (guarded in headers) without affecting production builds.
+    target_compile_definitions(kte_tests PRIVATE KTE_TESTS=1)
+
     # Allow tests to include project headers like "Buffer.h"
     target_include_directories(kte_tests PRIVATE ${CMAKE_CURRENT_SOURCE_DIR})