Operation-based undo/redo

The editor tracks changes with an operation-based history system rather than full-state snapshots, keeping memory usage predictable and low.

--Operation-based -- only the delta is stored per undo entry, not a full copy of the document.
--Ctrl+Z to undo, Ctrl+Shift+Z or Ctrl+Y to redo.
--History is capped at 50 entries to keep memory usage bounded.
--Consecutive typing in the same block is batched into a single undo entry for a natural editing feel.
--Multiple related changes (e.g., splitting a block) are grouped into a single batch operation.
--The redo stack is cleared when a new action is performed after undoing.