fix(sticky-headers): report the previous index in onChangeStickyIndex

[dennytosp]

Problem

onChangeStickyIndex is typed and documented as (current: number, previous: number) => void, but under the default configuration previous is always -1.

RecyclerView.tsx passed the currentStickyIndex state as the second argument, and that state is only written inside if (stickyHeaderHideRelatedCell). stickyHeaderConfig.hideRelatedCell defaults to false (RecyclerView.tsx:103), so for anyone who has not opted into hiding the related cell the state never leaves its initial -1.

Both the prop's JSDoc (FlashListProps.ts:419-424) and documentation/docs/fundamentals/usage.md:403-411 promise the previous index unconditionally, with no mention of hideRelatedCell.

Observed on main, logging the real callback arguments while scrolling past headers 0, 5 and 10:

[[0,-1],[5,-1],[10,-1]]      // expected [[0,-1],[5,0],[10,5]]

Fix

Track the last reported index in a ref of its own, so it advances on every sticky change regardless of hideRelatedCell. currentStickyIndex keeps its remaining job — hiding the cell behind the stuck header — and drops out of the sticky-header memo's dependency list, since the callback no longer reads it.

Test plan

Two regressions added to src/__tests__/RecyclerView.test.tsx: one for the default config, one for hideRelatedCell: true.

With the source fix stashed:

✕ reports the previously stuck index with the default config
    Received  1: 0, -1
    ->        2: 5, -1     (expected 5, 0)
✓ reports the previously stuck index when hideRelatedCell is enabled

The hideRelatedCell: true case passes on main on purpose — it pins the path that already worked so this cannot regress it.

With the fix:

yarn test        Test Suites: 14 passed, Tests: 189 passed, 189 total
yarn type-check  clean
yarn lint        clean
yarn build       clean

No device testing: this is a pure callback-argument contract change with no rendering effect, fully covered by the existing jest harness.