[shrinivas-sn]

What does this change?

• Shows a gradient at the top of the crossword clue list when it has been scrolled away from the start, to match the existing one at the bottom. It uses the same print and leftCol resets.
• Updates both gradients on every scroll and resize event instead of after a 100ms debounce, so they change while still scrolling.
• Moves the scroll and resize handling out of Layout into a small useScrollGradients hook in src/lib. It registers one handler for the life of the component and removes that same handler on unmount.

Closes #13543

Why?

The clue list only hinted that it could be scrolled down. Once you scroll, nothing shows there is more above.

The old effect also called libDebounce(...) again when removing the scroll and resize listeners. That creates a different function each time, so the listeners that were added were never removed. A single stable handler fixes that. There is no need to debounce: the state is two booleans, and React does not re-render when a setState call sets the value it already has.

The top gradient has pointer-events: none so it does not block clicks on the first clues.

How has this change been tested?

• New unit tests in src/lib/useScrollGradients.test.ts cover: bottom gradient only at the start, none when the list does not scroll, both gradients updating during a scroll with fake timers and no timers advanced, a few pixels from either end being ignored, resize updating the state, and one handler being added and the same one removed on unmount. With the old debounced add/remove put back into the hook, 4 of the 6 tests fail.
• pnpm tsc, eslint and prettier on the changed files, and stylelint on the component, pass.
• Full pnpm test: all suites pass except the cdk/lib/renderingStack.test.ts snapshots on my machine, where gu:repo comes from my fork's origin URL. That suite passes with origin pointed at guardian/dotcom-rendering.
• Not done: I have not run this in a browser, and Chromatic and Playwright have not been run. No existing story covers the crossword island, so I would be glad for a reviewer to check how the top gradient looks on the real page.

Screenshots

None yet, see above.