Preserve selected feature tab in Performance page exports (#9861)

* Preserve selected feature tab in Performance page exports

The Performance page tracked the selected feature tab via
`selectedFeatureTabIndex`, but it was never persisted to offline exports,
so loading exported data always reset to the first tab.

Mirror the Memory page's existing behavior: serialize the selected tab
index into `OfflinePerformanceData` and restore it when loading offline
data so the user lands on the tab they exported from.

Closes #5150

* Honor and clamp the restored tab index when loading offline data

Address review feedback on the offline tab restoration:

- The restored selectedFeatureTabIndex was immediately reset to 0 by the
  default-feature activation in TabbedPerformanceView. Use the restored
  index (clamped to the available tabs) when activating the default
  feature instead of hardcoding 0.
- The number of visible tabs can be smaller when loading offline data
  than when it was exported, so an out-of-range index could be passed to
  AnalyticsTabbedView. Clamp the index passed as initialSelectedIndex.
- Add a round-trip test for a non-zero selectedTab.

* Add release note for performance export tab fix