- Add middleClickDrag helper to CanvasHelper for simple MMB drag
- pan.spec.ts: use snapshot assertion + middleClickDrag helper
- multilineStringWidget.spec.ts: use snapshots, remove getCanvasOffset helper
- maskEditor.spec.ts: use dialog screenshot via expectScreenshot
- Move slot auto-node test to dedicated slotAutoNode.spec.ts with
type guards, beforeEach/afterEach, and getConnectionPos API