Some comments (useSticky.tsx, useSticky.test.tsx, RCA.md) still framed the
scrollbar-gutter/scrollBarStyles matching as the mechanism that prevents the
reported clipping, contradicting this branch's own hit-testing conclusion
that it's the unconditional width: maxWidth change that's load-bearing, with
gutter/style matching kept only as an unverified, secondary consistency
measure for combined horizontal+vertical scrolling. Rewrite those comments
and RCA sections to say so explicitly, and fix a getScrollBarSize.test.ts
comment that still described the removed maxWidth - scrollBarSize
subtraction as the header's "shrink amount".
Also remove the `as any` casts from the sticky regression test: type
`columns` as `Column<Row>[]` and pass `useSticky` directly, letting
`headerGroups`/`rows`/`cells` infer their existing react-table types (already
declared in `src/DataTable/types/react-table.d.ts`) instead of bypassing
type-checking of the exercised contract.
Refs #21063
sadpandajoe