fix: mark clearHistory as optional in MindElixirInstance

clearHistory is only registered by the operationHistory plugin, which is
skipped when allowUndo is false. Declaring it as a required method would
be a type-runtime mismatch. Mark it optional and update the JSDoc to
mention the allowUndo prerequisite.

Author: DisciplinedSoftware

src/types/index.ts

@@ -126,8 +126,10 @@ export interface MindElixirInstance extends Omit<Required<Options>, 'markdown' |
    * current diagram state. Call this after loading new data into an existing
    * instance (e.g. after `refresh()`) to prevent users from undoing back into
    * a previously loaded diagram.
+   *
+   * Only available when `allowUndo` is `true` (the default).
    */
-  clearHistory: () => void
+  clearHistory?: () => void
 
   selection: SelectionArea
   dragMoveHelper: ReturnType<typeof createDragMoveHelper>