Summary:

VirtualizedList/FlatList/SectionList re-run a small amount of
bookkeeping on every render and on every scroll event (60–120 Hz on
ProMotion displays). This PR removes three allocations from those hot
paths without changing any observable behavior:

1. VirtualizedList.render no longer builds a Set for
   stickyHeaderIndices on every render when the prop is not provided
   (the common case). The Set is now only created when the prop is
   present; the two .has() lookups use optional access.
2. ChildListCollection.forEach returns early when there are no nested
   child lists (the common case) instead of allocating a
   Map.values() iterator. This is called from _onScroll and the four
   other scroll callbacks on every scroll event.
3. _orientation() caches its result and only rebuilds the object
   when the horizontal prop changes. I18nManager.isRTL is a
   module-load constant (only changes on app reload), so the cache is
   invalidated solely by the horizontal prop. The object is replaced,
   never mutated, which keeps ListMetricsAggregator's field-based
   invalidation correct.

Changelog:

[GENERAL][CHANGED] - Reduce allocations in the VirtualizedList render and scroll path (avoid per-render Set allocation for stickyHeaderIndices, per-scroll-event Map iterator for the empty nested-list collection, and per-call orientation object allocation)

Test Plan:

• yarn test packages/virtualized-lists → 9 suites, 186 passed, 69
  snapshots:
  • ChildListCollection-test.js (new): forEach over populated/empty
    collection, removal, forEachInCell/anyInCell
  • VirtualizedList-test.js: stickyHeaderIndices not forwarded when
    the prop is absent (with ListHeaderComponent), forwarded when
    provided; orientation cache identity + invalidation on
    horizontal change
• yarn flow-check → 0 errors
• yarn lint → 0 errors, 0 warnings
• yarn format-check (changed files)

Micro-benchmark (Node v24, V8, 2M iterations, before vs after, same
machine; the real-world benefit is dominated by reduced GC pressure,
which is largest on low-end Android):