[audit] test-gap: multi-cursor undo/redo integration with ProseMirror history

[claude]

Problem

The multi-cursor plugin has 18 test files with good coverage of individual operations (typing, deletion, clipboard, arrow movement, soft undo). However, there is no integration test that verifies undo/redo behavior with ProseMirror's history() plugin.

What's missing

• Does Cmd+Z after a multi-cursor edit correctly undo all cursor changes as a single step?
• Does undo restore the MultiSelection correctly? (This depends on toJSON/fromJSON roundtrip, which is tested individually, but not through the history plugin path)
• Does Cmd+Z after multiple sequential multi-cursor edits produce correct incremental undo?

The toJSON/fromJSON and Selection.jsonID("multi", MultiSelection) registration are tested, but the actual history plugin integration (which serializes selections through this path) is not exercised end-to-end.

Suggested fix

Add integration tests that include ProseMirror's history() plugin in the test editor state and verify:

1. Multi-cursor edit → undo restores both document content and MultiSelection
2. Multiple sequential edits → incremental undo works correctly
3. Undo → redo roundtrip preserves MultiSelection with correct backward flags

Directory: src/plugins/multiCursor/__tests__/

[xiaolai]

Fixed in 464a96a.

Added undoRedoIntegration.test.ts with 5 integration tests using ProseMirror's history() plugin:

1. Undoes multi-cursor insert as a single step
2. Redo restores multi-cursor edit after undo
3. Multiple sequential edits produce incremental undo
4. Undo restores MultiSelection via toJSON/fromJSON roundtrip
5. Handles undo when multi-cursor edit merges into single range

All tests pass. MultiSelection's Selection.jsonID("multi", MultiSelection) registration enables proper history serialization.