App Router action queue: re-render final state when a pending action is preempted by navigation (fix stale data after revalidation)

## Upstream change

Next.js [1e113cf](https://github.com/vercel/next.js/commit/1e113cf5cdbb3662e858658dfd5d524af143d306) — "Fix stale data after navigation despite revalidation" (#95439)

## The bug

The App Router action queue always updates React state in dispatch order. When a navigation **displaces** (preempts) a pending server action, the navigation's promise is the last one dispatched. So when the preempted action later runs — even if it revalidated data — its resulting state is no longer reflected on screen, because the navigation's state was published last. Result: the UI shows stale data after navigation despite a successful revalidation.

## The fix

In `packages/next/src/client/components/app-router-instance.ts`:
- Add a `wasPreempted?: boolean` flag to `AppRouterActionQueue`.
- When a pending action is discarded because a navigation takes priority (`actionQueue.pending.discarded = true`), also set `actionQueue.wasPreempted = true`.
- In `runRemainingActions`, once the queue is idle (`pending === null`) and `wasPreempted` was set, clear the flag and publish the final queue state: `startTransition(() => setState(actionQueue.state))`. The existing `needsRefresh` flush now runs alongside this.

## Why it's relevant to vinext

vinext reimplements App Router client-side navigation and the server-actions dispatch queue. If vinext's action queue does not re-render the final state after a preempting navigation, users will see stale data after a navigation that interrupts a revalidating action — matching the upstream bug.

## Action

- Check vinext's App Router action-queue implementation for equivalent preemption handling (navigation displacing a pending action).
- Ensure the final queue state is re-published once the queue drains after a preemption, so revalidated data is reflected.
- Port the regression test (`actions-discarded-navigation-revert`) which was red before the fix.

## References

- Commit: https://github.com/vercel/next.js/commit/1e113cf5cdbb3662e858658dfd5d524af143d306
- PR: https://github.com/vercel/next.js/pull/95439

Inspect vinext's App Router action-queue implementation for navigation preemption, then run the existing actions-discarded-navigation-revert regression test if present. Compare its behavior with the upstream change and verify that, after a navigation displaces a pending action, the final queue state is re-published when the queue drains and the regression test passes.

Written by the indexing model from the issue text.