Exiting a subgraph via undo requires an extra undo step (pinned in changeTracker.spec.ts since 2026-07-22, untracked)

## Summary

Exiting a subgraph via undo requires one more undo step than expected. The behavior is pinned inside a browser test comment but has no tracking issue.

`browser_tests/tests/changeTracker.spec.ts`, test `Does not restore invalid navigation stack` (added in #13327, 2026-07-22), ends with:

```ts
await undoAndWait()
await undoAndWait()
await expect
.poll(
() => comfyPage.subgraph.isInSubgraph(),
'Currently bugged: click after subgraph requires additional undo'
)
.toBe(false)
```

The test needs two undo calls where the user-visible expectation is one. The "Currently bugged" annotation has lived only in that poll message since 2026-07-22; searching open issues for it returns nothing.

## Expected

One undo step after a subgraph conversion sequence returns the canvas to the parent/root scope, matching the number of user actions taken.

## Where

- `browser_tests/tests/changeTracker.spec.ts` final poll of `Does not restore invalid navigation stack` (present on both `main` 296fc5cd07 and `feature/ecs-migration` f1bfb313d6, verified 2026-08-23)
- Undo path: `ChangeTracker.updateState` replays serialized JSON via `app.loadGraphData`; navigation stack validation happens in `restore()` only on workflow switch, not on undo

## Notes

Found during an undo-coverage audit in the ECS migration program (task ax-12). This is a main-line behavior, not introduced by the ECS branch. If a fix lands, the test's double `undoAndWait()` and the poll message should be updated in the same PR so the characterisation does not fossilise the defect.