Nord theme and undo system refinements

- Improve text input/event batching
- Enhance debugging with optional instrumentation
- Begin implementation of non-linear undo tree structure.

UndoSystem.h

@@ -2,6 +2,8 @@
 #define KTE_UNDOSYSTEM_H
 
 #include <string_view>
+#include <cstddef>
+#include <cstdint>
 
 #include "UndoTree.h"
 
@@ -10,7 +12,7 @@ class Buffer;
 
 class UndoSystem {
 public:
-	explicit UndoSystem(Buffer &owner, UndoTree &tree);
+    explicit UndoSystem(Buffer &owner, UndoTree &tree);
 
 	void Begin(UndoType type);
 
@@ -28,23 +30,28 @@ public:
 
 	void discard_pending();
 
-	void clear();
+ void clear();
 
-	void UpdateBufferReference(Buffer &new_buf);
+ void UpdateBufferReference(Buffer &new_buf);
 
 private:
-	void apply(const UndoNode *node, int direction); // +1 redo, -1 undo
-	void free_node(UndoNode *node);
+    void apply(const UndoNode *node, int direction); // +1 redo, -1 undo
+    void free_node(UndoNode *node);
 
-	void free_branch(UndoNode *node); // frees redo siblings only
-	UndoNode *find_parent(UndoNode *from, UndoNode *target);
+    void free_branch(UndoNode *node); // frees redo siblings only
+    UndoNode *find_parent(UndoNode *from, UndoNode *target);
 
-	void update_dirty_flag();
+    // Debug helpers (compiled only when KTE_UNDO_DEBUG is defined)
+    void debug_log(const char *op) const;
+    static const char *type_str(UndoType t);
+    static bool is_descendant(UndoNode *root, const UndoNode *target);
+
+    void update_dirty_flag();
 
 	Buffer *buf_;
 	UndoTree &tree_;
-	// Internal hint for Delete batching: whether next Append() should prepend
-	bool pending_prepend_ = false;
+    // Internal hint for Delete batching: whether next Append() should prepend
+    bool pending_prepend_ = false;
 };
 
 #endif // KTE_UNDOSYSTEM_H