feat: expose clearHistory() method on MindElixir instance

Adds a clearHistory() method to the operationHistory plugin that
resets the undo/redo stack to an empty state with a fresh baseline
snapshot. This is needed by hosts that load new data into an
existing MindElixir instance (e.g. via refresh()) and want to
prevent users from undoing back into the previous diagram's state.

Author: DisciplinedSoftware

src/plugin/operationHistory.ts

@@ -88,6 +88,11 @@ export default function (mei: MindElixirInstance) {
       }
     }
   }
+  mei.clearHistory = function () {
+    history = []
+    currentIndex = -1
+    current = mei.getData()
+  }
   const handleOperation = function (operation: Operation) {
     if (operation.name === 'beginEdit') return
     history = history.slice(0, currentIndex + 1)